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ZOOLOGY

GALLOWAY

is TON'S SCIENCE SERIES

ZOOLOGY

A TEXT-BOOK FOR

SECONDARY SCHOOLS, NORMAL SCHOOLS
AND COLLEGES

BY

THOMAS WALTON GALLOWAY, Pn.D.

PROFESSOR OF BIOLOGY IN THE JAMES MII.I.IKIN UNIVERSITY
DECATUR, ILLINOIS

Seconfc

240 Ullustrations

PHILADELPHIA
P. BLAKISTON'S SON & CO.

IOI2 WALNUT STREET
1909

COPYRIGHT, 1906, BY P. BLAKISTON'S SON & Co.
COPYRIGHT, 1909, BY P. BLAKISTON'S SON & Co.

PRESS OF
LANCASTER. PA.

PREFACE TO THE SECOND EDITION.

The author desires to express his obligation to the teachers
whose approval has made necessary, thus early, a second edi-
tion of the work. The success of any text is, on last analysis,
due to the insight of the teacher who uses it. The writer will
be glad to welcome, from teachers, suggestions for later
editions.

The new edition makes it possible to remove some typo-
graphical and other errors, and to make certain minor changes
and additions which it is hoped will render the book more
acceptable.

A slight change is made in the title; and the publishers
announce a reduction in the price.

T. W. GALLOWAY.

PREFACE TO FIRST EDITION

Many attempts have been made in recent years to determine
what presentation of Zoology (and Botany) is suitable for a
first course, especially in the secondary schools. The follow-
ing- principles may be said to represent the main points of
agreement among teachers :

1. The work done in a first course is primarily for pupils
who do not take a second course. This first course should be
handled, therefore, as a life-training rather than as satisfying
an admission requirement.

2. Laboratory work and field work are essential, both to
proper interest and to proper results, and should not be merely
illustrative of text or lecture work, but as far as possible should
be the foundation and point of departure of the lectures and
the text. No instrumentality open to the teacher is better than
the laboratory as a means of securing mental growth for the
pupils.

3. On the other hand it is equally important that the work
shall not be confined to the field and the laboratory. " There
are many things in the infinite concourse of particulars which
we cannot afford to verify by experiment." The end of labo-
ratory work is gained for the elementary student when he
comes to appreciate the method and spirit by which sound in-
vestigation proceeds, has acquired enough technical skill to fol-
low elementary investigation on his own behalf, and has
learned how to appreciate, and if necessary to verify, the state-
ments of others. It is as easy to waste time in the laboratory
as in reading text-books.

4. Laboratory directions in texts should be suggestive rather
than exhaustive. They should arouse the interest of the stu-
dent and direct his investigations rather than give him the
information which he seeks.

PREFACE. Vll

5. Work in Zoology, once begun, should continue for a
whole year if possible.

If we may judge from the diversity of texts which have ap-
peared in the last fifteen years, there are some important points
yet undetermined. Some of these are :

. i. What Zoology is of most worth? Which of the numerous
divisions of the subject should receive greatest stress in a first
course? Should a preponderance of attention be given to the
study of structure and to dissections ? Or to a comparison of
the various ways of performing the functions necessary to all
animals ? Or to the study of the classification of animals into
their families and species ? To the economic value of animals ?
Or to the study of their relations to each other, and to the plant
kingdom, and to the inorganic environment? Text-books of
the last fifteen years have passed through several distinct
phases in the effort to find an answer to these questions.

2. What proportion of such a course should be given to the
descriptive and theoretical, and what to the practical or labo-
ratory aspects of the subject?

3. What should determine the order of presentation of the
subject, logic or expediency? and what is expedient?

The present book is intended as a suggestion in the direction
of an answer to some of these questions. The plan of treat-
ment here recommended has been followed by the author in
his own classes for a number of years. By its use he has
secured good interest and fine spirit, in the study of animals
and animal life, on the part of beginners ranging from the
third year of the preparatory school to freshmen in the college.

The following principles have guided in the selection and
the arrangement of the material of the present volume :

i. A first course should really be a foundation course, and
as such should give the student a broad and catholic view of
the whole subject, without thereby becoming commonplace. It
should utilize all the main departments of Zoology, because
each department contains matter which should be familiar to
all persons of ordinary education. Furthermore, the depart-

Vlll ZOOLOGY.

ments of morphology, physiology, ecology, distribution, and
classification furnish exercises which have distinct, 2^6. yet com-
plementary, pedagogical value. Any single phase of the sub-
ject, however important or interesting, gives a false and there-
fore an unscientific view of the wonderful science of Zoology,
unless it is supplemented by the others. Therefore the same
book, if it is to serve the pedagogical needs of beginners,
should contain fairly representative matter from all the main
departments of the science ; and it should at the same time pro-
vide both for the descriptive work and for the practical work
in the field and laboratory.

2. The time in an elementary course should be about equally
apportioned (i) to laboratory work (chiefly in physiology and
in the larger problems of morphology rather than in minute
dissection) ; (2) to field observation on physiology, life his-
tories, and the simpler problems of distribution and life rela-
tions; (3) to the body of the descriptive text; and (4) to
classes of questions demanding reference to classical zoological
authorities.

3. The matter of greater native interest should underlie and
sustain that of less. It should not, however, exclude or efface
the latter. The most interesting is often the least important.

4. The student must not be taught that observation is the
only source from which he may draw. Too much of this im-
pression has arisen from the necessary appeal for more and
better laboratory work. It has become quite as necessary for
him to know something about authorities (a word formerly
in some disrepute among scientists, but now of increasing im-
portance). Hence even a beginner's course in natural history
should make large demands upon the student in the matter of
library work, both as more economical of his time and, on
the whole, likely to be more accurate than his own uncorro-
borated observations. The interaction of authority and in-
dividual discovery furnishes the teacher his supreme oppor-
tunity in the development of the student.

5. Certain of the general facts and principles which the

PREFACE. IX

beginner cannot be expected to discover for himself should be
presented early, in order to give the student a skeleton or
dimensions, so to speak in which he shall later insert the
particulars which he discovers. He must have this in order to
unify his own results in the brief time at his disposal. The
lack of this unifying result is the ground of the just complaint
concerning much of the unorganized and unrelated laboratory
instruction in the secondary schools.

6. While it is necessary to bring our materials from various
departments of Zoology and is desirable that the student should
be able to recognize whether a given problem is primarily one
of structure or function or relation, the total result of an
elementary course of Zoology should be a sense of unity, of
continuity, and of interdependence. The final view of the
student should be of life and organic progress, and not of a
disjointed science, dissected in the house of its friends.

7. The teacher should have some latitude in the choice of
matter and emphasis, in order that both may be properly suited
to his equipment and locality. It should be impossible for the
teacher or the class to use a text-book in a slavish, or parasitic
fashion. Therefore a text-book should contain and suggest
much more than one teacher or one class can use in the time
allowed. This not only gives the teacher a chance (and makes
it necessary for him) to mould his own course, but causes the
student to realize that he is a mere beginner when he has com-
pleted his first course.

In attempting to apply these principles to the present book
the author has made use of the following devices :

i. The book is divided into two portions: (i) a general
part dealing largely with broad biological problems and princi-
ples, which constitute the foundations of the science and are
felt to be for the most part, beyond even the verification of the
elementary student (chapters I-VIII) ; and (2) a special part
(chapters IX-XXIV), in which the various principal phyla
of animals are taken up in succession, beginning with the
lowest. The purpose has been to make this part particularly

X ZOOLOGY.

illustrative of the principles laid down in the general portion.

2. Each chapter of the general part contains the following
elements: (i) the general statement of principles or facts;
(2) interspersed with this are such practical exercises for labo-
ratory, field, or library, as have been found practicable for
elementary classes. These are intended to compensate for the
enforced brevity and abstractness of definitions and description,
by causing the student to find concrete illustration of the princi-
ples; (3) an analytic summary of the most important general
truths of the chapter in outline, at the close of the chapter;
and finally (4), a list of supplementary topics for individual
laboratory or library investigation and report. These supple-
ment and illustrate the text, and enrich the review by introduc-
ing a new view-point and new matter.

3. In the chapters of the special part each phylum is intro-
duced by field and laboratory work on some representatives
taken as types. This is followed, corrected and enlarged by a
brief discussion of the typical condition of the organs and
functions in the group as a whole. This serves to unify the
isolated and local observations of the student. Next follows
a brief statement of the most important facts of classification,
together with ecological and economic suggestions. Finally,
each chapter concludes with a list of supplementary questions
calling for field, laboratory, and library work in review, and as
a brief view of new material.

4. The figures are carefully selected, the majority of them
being specially made for this book. With each figure of special
moment is a brief list of queries designed to assist the student
in the study of the figure. It is a common complaint among
teachers that it is difficult to get students to appreciate and to
use illustrations intelligently.

5. The concluding chapter consists of practical questions
and special exercises which necessitate a review by the student
of all that is essential in the book, from a new point of view.

6 A briefer course may be secured by the omission of the

PREFACE. XI

matter in fine print, which is intended only for such schools as
can give a full year to the subject of Zoology.

7. The headings of paragraphs are printed in black-faced
type, in order to emphasize the analysis of subject matter.
Technical terms are in italics the first time they appear. The
author does not agree that all technical language should be
omitted from even an elementary course.

The author extends most cordial thanks to the many pub-
lishers and authors whose courtesy enables him to reproduce
classic illustrations from their copyrighted works. Especially
to be mentioned are Macmillan & Co., D. Appleton & Co.,
Wm. Blackwood & Sons, Adam and Charles Black, Swan
Sonnenschein & Co., Henry Holt & Co., Houghton, MifHin &
Co., The Open Court Publishing Co., N. G. Elwert, Leipzig;
Dr. Carl Chun, Leipzig; The Division of Publications, Wash-
ington, D. C, University of Minn. Agricultural Experiment
Station, Dr. A. Agassiz, Dr. George Dimmock, Dr. Henry
C. McCook, Professor G. H. Parker. Recognition is given
to the sources in immediate connection with the figures.

The thanks of the author are also due to many fellow teach-
ers for suggestions and criticisms during the progress of the
work ; but especially to Dr. Frank W. Bancroft of the Univer-
sity of California and Professor J. H. Gerould of Dartmouth
College who made extensive criticisms and suggestions while
the book was in manuscript, and to Dr. J. W. Folsom of Illinois
University to whose skill and painstaking is due whatever
merit the original drawings may possess. Many of the origi-
nal photographs were also made by Dr. Folsom.

T. W. GALLOWAY.

JAMES MILLIKIN UNIVERSITY.

TABLE OF CONTENTS

CHAPTER PAGE

I. INTRODUCTION 2

II. PROTOPLASM : ITS MORPHOLOGY AND PHYSIOL-
OGY 8

III. THE ANIMAL CELL : ITS MORPHOLOGY ANC

PHYSIOLOGY 1 8

IV. FROM THE SIMPLE CELL TO THE COMPLEX

ANIMAL 27

V. CELLULAR DIFFERENTIATION : TISSUES 40

VI. GENERAL ANIMAL FUNCTIONS AND THEIR AP-
PROPRIATE ORGANS 59

VII. PROMORPHOLOGY 83

VIII. INDIVIDUAL DIFFERENTIATION AND ADAPTATION 92

IX. A GENERAL PREVIEW OF THE ANIMAL KINGDOM. 135

X. THE PROTOZOA 142

XL THE PORIFERA 156

XII. THE CCELENTERATA 165

XIII. THE UNSEGMENTED " WORMS " 183

XIV. THE ECHINODERMATA 2OO

XV. THE ANNULATA: SEGMENTED "WORMS"... 216

XVI. THE MOLLUSCA 234

XVII. THE ARTHROPODA 261

XVIII. THE CHORDATA: PROTO-VERTEBRATA 308

XIX. THE CHORDATA : VERTEBRATA 312

XX. PISCES 355

XXI. AMPHIBIA 372

XXII. REPTILIA 380

XXIII. AVES 394

XXIV. MAMMALIA 427

XXV. GENERAL SUMMARY: A REVIEW OUTLINE 45 l

APPENDIX. SUGGESTIONS TO TEACHERS 455

INDEX 47

xii

ZOOLOGY

CHAPTER I.
INTRODUCTION.

1. Nature presents to man, as he looks upon it, a great and
interesting variety of material objects. Each member of the
race gathers in his lifetime, by means of experience and infer-
ence, a certain limited knowledge of these objects and of the
changes to which they are subject. The knowledge, thus col-
lected and systematized in the course of the history of the
human race, constitutes the so-called Natural Sciences. Every
one of us, whether he deliberately chooses or not, must be in
some degree a natural scientist. The beauty and interest of
the work has attracted and charmed thousands of people of
all conditions, in all parts of the world.

We commonly speak of material objects as either living or
non-living as organic and inorganic. The study of living
things in all their relations we call Biology. Physics and Chem-
istry are often considered as dealing exclusively with inor-
ganic matter, and are therefore placed in contrast with Biology.
Their principles apply, however, in the realm of living things
just as truly as in the non-living, and one must not imagine
because of this antithesis that the phenomena of life can be
explained apart from chemical and physical laws. The term
Biology was first introduced about the beginning of the nine-
teenth century, and is intended to express the fact that plants
and animals are similar in their most essential structure and
activities. The term Natural History is sometimes used
synonymous with Biology.

2. Zoology. Owing to the fundamental likeness of all
living matter, there is great theoretical difficulty in distinguish-
ing between the plant and animal kingdoms. The practical

2 ZOOLOGY.

difficulty however is confined to the very lowest and simplest
forms of life. The plants and animals which come under the
common observation of the student are readily distinguished.
It is only the deeper study which reveals the underlying simi-
larity of all living objects. The branch of Biology which
treats of plants is called Botany; that which deals with ani-
mals, Zoology.

3. Purpose of Zoological Study. The study of zoology
is valuable to the general student because animals constitute
one of the most interesting and important features of our sur-
roundings, and have a most vital bearing upon our well-being.
In the second place, it adds to our knowledge of the structure
and activities of man himself, to study him in his proper rela-
tion to other animals. Finally, its study demands of the student
the use of the scientific method, which consists of observation
of as many facts as possible at first hand, of comparing and
contrasting these facts with one another and with the observa-
tions of others, and of reaching such conclusions from them
as may seem legitimate. In common with the other natural
sciences it is thus seen to have high educational value, apart
from the practical importance of the knowledge itself.

To the investigator, the ultimate object of zoological study
is to find the real nature of animal life as it exists, the mode of
its development, and the causes which have brought it to its
present exquisite variety and adjustment. These larger and
more general questions constitute what may be called theor-
etical Zoology, or the principles of Zoology.

4. Practical Exercises. Cause the student to select ten or more kinds
of wild animals with which he is partially acquainted, and, from his obser-
vation and experience, to enumerate the points at which they touch human
welfare. Are they, in each instance, to be classed as helpful ? as harm-
ful ? or merely as indifferent ? Is their influence upon man's interest direct
or indirect?

What animals have, in the past, most appealed to your interest? Select
that particular quality in which you have been most interested (structure,
powers, instincts, habits) and show how the attempt to study or explain
any one takes you at once into all the others.

5. Divisions of the Science. The facts and principles
which have been, and are yet to be, discovered concerning ani-

INTRODUCTION. 3

mals are so numerous and various in their bearings, and in-
vestigators approach the subject from such different points of
view that it is necessary, in order to express these results, to
divide zoology into several branches or departments. It must
be held in mind, however, that these divisions are more or less
artificial, and that the facts of each department are to be con-
sidered in connection with those of all the others, if they are
really to be understood. With all its departments, animal life
is to be thought of as a whole. Structures exist for the per-
formance of function, and the activities are intended to adjust
the animal to its whole life-relation.

6. Morphology is the branch of the science which deals
with form or structure in its broadest sense, whether internal
or external, partial or total. In its most general sense it em-
braces the study of animals from the standpoint of symmetry,
that is, the form of the organism with reference to certain
planes passing through the body. For example, the human
body may be so divided by a single plane that two essentially
similar parts result, the right and the left. Again similar
parts may succeed each other in a linear series, as in the seg-
ments of the earth-worm ; or they may radiate from a central
point, as in the arms of the star-fish. This is the most funda-
mental kind of morphology. It relates the organism to space.
It is called Promorphology, and is related to Zoology some-
what as the study of crystals is to Mineralogy.

Anatomy is that department of morphology which treats
of the structure of parts of the individual, as the organs and
systems of organs, the tissues, the cells, and so forth. This is
known as gross anatomy if the study pertains to the larger
units, as organs ; it is called Histology, if the constituent ele-
ments of these organs (as tissues and cells) are to be con-
sidered.

Thus far we have thought of structure as stationary or per-
manent. As a matter of fact we know that each organism
begins life in a very modest way, as a single " cell," and grows
more complex by fairly well-defined stages until the adult con-

4 ZOOLOGY.

dition is reached. The science of Embryology is the record of
this history of the successive stages which the individual ani-
mal assumes in becoming adult, or at least until its organs are
essentially formed.

7. In Physiology are considered the facts and laws relating
to the activities or functions of the organism and of its separate
parts. It includes the tracing back of the adult activities to
their lowest form, as found in the simplest animals or the
youngest stages of the higher animals. It includes the powers
of the single cell; the chemical and physical processes which
seem to underlie all the functional activities; the division and
more perfect performance of the primitive functions as the
various organs arise and come to do their, special work.
Finally it includes the relation of the animal as a whole to other
animals of the same or of different species, to plants, and to
the inanimate surroundings. The term Ecology is applied to
this branch of physiology which treats of the relation of the
organism to the complex and wonderful conditions in which
it finds itself. Of recent years much emphasis is being given
to this branch of zoology.

8. Animals may be studied as to their distribution or occur-
rence in the world. For example, we find lions in Africa and
Asia only, and the African and Asiatic lions are of different
varieties; the giraffe is found only in Africa; man is found
over the most of the habitable globe, but before the era of easy
communication between distant countries the men of different
regions were conspicuously different. Again we can easily see
that the animals that live in the various bodies of water are
very different from those living on the land ; those in the frigid
zones are different from those in the temperate and torrid.
All such topics are treated under the head of distribution, 01
geo graph ical distributio n .

This is distribution in space. Similarly the various systems
of rock-strata are characterized by more or less different fossil
remains, indicating a variation in the animal life during the
successive periods of the earth's history. This distribution of

INTRODUCTION. 5

animals in time is the subject-matter of P alee o zoology. The
facts of palseozoology and the conclusions resting thereon are
among the most important in the whole realm of Zoology,
inasmuch as they supplement the facts gained from the study
of embryology and morphology of living species, thus enabling
the investigator to trace the history of the various races of
animals into the remote past.

9. Practical Exercises. Let the student submit a written report on
the distribution of the animals in his immediate neighborhood, based on
his own observations. The report need not be exhaustive in order to
convince the student of the effect of the environment, which includes
everything in the surroundings, on the distribution of animals. Some
classification should be made of the varieties of territory included ; as
river, pond, lowland, woodland, prairie, mountain, and the like. Deter-
mine, by reference to the authorities available, the geographical distribu-
tion of the following: the elephant, the camel, the kangaroo, the horse,
the white bear, the seal, the salmon, the crocodile, the reef-forming coral,
the sponge of commerce.

10. Classification. In studying animals and plants one is
soon impressed with the fact that among the thousands of
individuals, even of the same general kind, there are no two
exactly alike; and yet among them all, with their manifest dif-
ferences, there are numerous points of similarity. These two
facts make it possible to group those most alike into more or
less coherent classes, separating them at the same time from
other classes. The forming, naming, and defining of these
groups and subgroups we call Taxonomy or Classification.
Manifestly, true classification must depend upon the facts de-
rived from the completest possible study of the structure and
relations of organisms, and can only be perfect when we know
all that is to 'be known about them. In addition to' displaying
our present knowledge of the relationship of animals, classifi-
cation serves a most important end in giving us more rapid
power of using that knowledge in getting further knowledge
that is needed.

11. Historical. Zoology as a science can scarcely be said
to be more than three hundred years old, although Aristotle,
more than three hundred years before Christ, wrote much of

O ZOOLOGY.

value concerning animals. Later many facts of general anat-
omy were discovered in connection with the study of medicine,
and about 1600 the invention of the microscope opened up the
field of histology. Toward the end of the seventeenth century
an effort was made to establish a scientific classification of
animals. Since that time very much of the attention of stu-
dents of zoology has been turned in this direction. During
the last century however there has been a constantly increasing
interest in the study of embryology, of histology, and in the
general theoretical questions, the answers to which depend
on the bringing together of the results of studies in all depart-
ments. Such are the problems of race development or evolu-
tion, of heredity, of man's place in nature, and the like. The
most notable development of the subject in recent years has
been in connection with the study of the finer structure of the
cell, in more exact methods of studying physiology, and in
extending its scope to take in the lower organisms as well as
the higher and the single cell as well as the organs. It is
important to add that all this work is now being done in a
comparative way. The necessity of comparing the histology,
the embryology, and the physiology of one animal with that
of another arises from the belief in the unity of animal life,
and that all animals are really akin. If animals of different
kinds are really related, their likenesses and differences take on
a new meaning to the student, and classification comes to ex-
press the degree of kinship, as well as to serve the convenience
of the investigator.

1 1 a. Practical Exercises. By reference to text-books ar-
range a list of the most important zoological discoveries, by
centuries. What great contributions were made by the follow-
ing men? Harvey; the Janssens; John Ray; Linneus; La-
marck; Cuvier; Schleiden; Schwann; von Baer; Charles
Darwin ; Wallace ; Louis Agassiz ; Huxley ; Weismann. When
and where did these scientists live? Mention five American
zoologists and indicate their chief work.

INTRODUCTION. 7

12. Summary.

I. Natural Science embraces :

A. The sciences of inanimate things

Astronomy,
Geography,

Meteorology, Mineralogy, Lithology, etc.

B. The sciences of animate things

Botany,

Zoology.

(Physics and Chemistry are fundamental to both
groups of sciences; Geology embraces portions of
the subject-matter of both groups.)

II. Subdivisions of Zoology.

A. Morphology:

1. Promorphology, which treats of general
form;

2. Anatomy ; = the structure of parts:

Gross = structure of organs and systems

of organs ;
Microscopic = (Histology, Cytology) ;

structure of tissues and cells ;

3. History of Development (structural stages) :

Individual = (Embryology, Ontogeny) ;
Racial = (Phylogeny).

B. Physiology :

1. Physiology proper ; = the functional relation
of part to part and to the whole.

2. Ecology ; = relations of the individual to its
whole surroundings.

C. Distribution :

1. In space = (Geographical Distribution) ;

2. In time = ( Palaeozoology, as revealed by
fossils) ;

D. Classification, or the grouping of animals ac-
cording to their likeness or kinship.

CHAPTER II.

PROTOPLASM: ITS MORPHOLOGY AND PHYSIOLOGY.

13. Life. Life may be thought of in two somewhat dis-
tinct ways. It may be considered, first, merely as an expres-
sion for all the various activities of the organism, the sum
of all the phenomena of its existence ; or, second, as a force or
form of energy from which the special modes of activity, as
feeding, growth,' motion, and thinking, arise. The latter is
the more common use of the term, and yet the former is the
only use of it which can be completely justified. Much of the
activity of living things may be explained by reference to the
ordinary physical and chemical laws. At least we know that
these latter processes underlie all the actions which we call
vital. It is indeed a question whether all vital phenomena are
not finally to be explained by means of them, without the need
of assuming any special vital principle or force. There seems,
however, a growing disposition among scientists to admit that
the action of chemical force does not suffice to explain all the
phenomena of the living animal. Whether this is true or not,
it is often convenient to speak of " vital force " as if it were a
cause embracing more than is usually included in the knoiun
chemical and physical actions.

14. The Relation of Protoplasm to Life. Whatever life
may be, in the last analysis, we never observe its manifestations
except in connection with a substance called protoplasm, which
is found both in plants and animals. Protoplasm does not con-
tain any chemical elements which are not found in other than
living materials. Notwithstanding this fact, protoplasm is
different from any other known substance. It is more com-
plex and more highly organized, as to its machinery, than any
other chemical or physical compound whatsoever. Protoplasm
has the power of growing by taking up and changing other

PROTOPLASM. 9

non-living substances ; but, so far as we know, it is never pro-
duced except as the result of the growth and division of
antecedent protoplasm. The protoplasmic or living material
in an organism is normally composed of a number of unit-
masses called cells (see Chapter III). These unit-masses of
protoplasm are in some degree independent of one another,
because normally each tends to form a wall about itself; and
yet it is highly probable that the whole protoplasm of an ani-
mal is physically continuous by means of delicate connections
between the units. The life of the cells is not quite the same
thing as the life of the organism to which they belong, for
in animals composed of more than one cell a cell may die with-
out involving the death of the animal. The protoplasm of the
cell may also retain life for a time after separation from the
living animal or after the animal as a whole has ceased to live.
This may be seen in the fact that the colorless corpuscles of
the blood, which are regarded as cells of the body, may con-
tinue to move and show other evidences of life after removal
from the body.

15. Protoplasm. The protoplasm is the strictly living,
active material of organisms. The term is sometimes applied
so as to embrace non-living substances which are found in
close connection with that which is thought to be " alive."
Even limiting the term protoplasm to the living matter, we
must avoid regarding it as a single substance of definite com-
position. We should rather consider it a very complex mixture
of substances, each of which is a highly complex compound.

While protoplasm seems fundamentally the same in plants
and in animals, there are yet important differences; and it is
probable that the living matter of different animals, and even
of different parts of the same animal, is diverse in chemical or
physical structure. This fact, rather than any possible dif-
ference in the " life-force " itself, seems responsible for the
diversity of powers of different organisms and of different
organs.

10 ZOOLOGY.

16. Chemical Composition of Protoplasm. It is impossible to make
a satisfactory chemical analysis of protoplasm, as it loses its characteristic
powers and probably undergoes important chemical and physical changes
in the act of analysis. The dead material thus obtained is no longer the
substance with which we started, either as to its power or its structure.
The experiment shows however that the substance is both chemically and
physically unstable. By an analysis of the dead protoplasm, we find pres-
ent several complex organic compounds, known as proteids, carbohydrates
(starches and sugars), fats, ferments, pigments, etc. In addition to these
are simpler inorganic compounds, as water and various salts. Doubtless
some of these materials are food-substances on their way to form proto-
plasm, and others are the waste-products of protoplasmic disruption, ready
to be cast out of the cell. The proteids are the most complex of all these
substances and it is believed that protoplasm finds- its real basis in these.

The proteids are various in composition and properties, but agree in
that their molecules contain carbon, hydrogen, oxygen, nitrogen, and sul-
phur, in proportion roughly as follows : C 53%, O 22%, N 17%, H
7%, S i%. The white of egg, the fibrin of the blood, and casein in milk
are examples of proteid.

Carbohydrates consist of C, H, and O. The latter elements are always
present in the ratio in which they are represented in water (H 2 O), e. g.
CeHioCX The starches, sugars, and cotton fibres are illustrations.

The fats contain the same elements as starch, but the percentage of
oxygen in terms of the hydrogen is much smaller than in the starches.

The ferments are complex organic substances which have the power of
producing important chemical changes in other substances without being
themselves consumed. They play an important, but not thoroughly un-
derstood, role in the activities of the organisms, both within and outside
the cells which produce them. The active principle of the digestive juices,
as ptyalin and pepsin, are examples of ferments which have been extruded
from the cells.

Water (H 2 O) is very important in both the chemical and physical
structure of protoplasm. It is very variable in amount, and the degree
of activity of the protoplasm is roughly proportional to the amount of
water present. Traces of inorganic salts, compounds of chlorine, potas-
sium, sodium, calcium, phosphorus, iron, etc., are als6 found in solution
in the water.

17. The Physical Structure of Protoplasm. This varies
much from time to time. On account of differences in the
amount of water present, the consistency of protoplasm may
vary from the quite fluid condition found in actively growing
parts, to the very much more solid condition apparent in dry
seeds and in the resting or encysted stage of some animals.
In these latter instances the protoplasm eliminates a large per

PROTOPLASM. II

cent, of its water, forms a thick wall, and thereby becomes en-
abled to resist drouth and heat and cold as it could not possibly
do otherwise. Under ordinary circumstances protoplasm ap-
pears as a semi-fluid or gelatinous material.

Concerning the architecture of protoplasm there is much
diversity of opinion. It seems probable that this, like the
chemical composition, is subject to considerable variation. It
is certainly very complicated and represents at least two physi-
cally distinct substances mingled in a very effectual and won-
derful way. In some cases at least these take on the appear-
ance of a foam structure such as is obtained in an emulsion
of oil in water, or of air and water in a soapy lather. What-
ever its form may be, it seems that there must be a close rela-
tion between the architecture and the powers which protoplasm
shows.

1 8. Physiology of Protoplasm. The mass of protoplasm
which we have called a cell, or unit, performs practically all
the functions shown by the more complex organism. It has
the power of feeding, of growth, of reproduction, of motion in
response to stimuli. Even in the higher animals, made up of
many of these units, the processes are performed, on last
analysis, by the individual protoplasmic units of which the
body is composed.

19. Irritability. Owing to its chemical and physical in-
stability, living protoplasm is constantly changing. These
changes may be the direct result of internal or external con-
ditions to whose influence the protoplasm may respond by a
manifestation of energy greater than that involved in the
stimulus. This quality is called irritability. It further seems
that changes may originate within the protoplasm itself,
though this is much more difficult to demonstrate and may
merely represent our ignorance of the processes occurring in
the protoplasm. This power is called automatism. These are
the most fundamental qualities belonging to protoplasm, and
serve to make possible those which follow : viz., motion, assimi-

1 2 ZOOLOGY.

lation, growth, etc. Protoplasm varies in the degree of irri-
tability. In general it responds to stimuli most normally under
those conditions which are most favorable to the ordinary vital
processes.

20. Stimuli. All the disturbing forces or conditions, ex-
ternal or internal, which tend to cause response in living proto-
plasm, are called stimuli. The principal stimuli are, chem-
ically active substances, moisture, contacts, heat, light, elec-
tricity, and gravity. Inasmuch as irritability lies at the
foundation of the various protoplasmic activities mentioned
below, all the natural causes which modify irritability, also
modify, through it, the vital processes, such as motion, growth,
etc.

Light affects protoplasm profoundly. The direction of motion in pro-
toplasm is largely determined by light. Light may either attract or repel
protoplasm. Excess of light retards growth. Heat strongly modifies the
rate of all the vital processes. There is an optimum temperature at which
the protoplasm best performs its work. An excessive increase or decrease
of this temperature produces a cessation of activity, a condition of rigor,
and death. The fatal maximum temperature for ordinary animal proto-
plasm may be said to be about 45 or 50 C. ; the minimum, o, or below.
Chemical agents may stimulate protoplasm in such a way as to attract or
repel organisms. Paramecia, which are single-celled animals, may be seen
to gather about an air-bubble, or at the margin of the cover-glass. They
will retreat before an encroaching solution of certain salts.

It is a most significant fact in this connection that protoplasm may
become, so to speak, accustomed to a stimulus which has been long con-
tinued, so that it ceases to respond in the customary way. Protoplasm
may gradually be brought, for example, to endure and thrive at a tem-
perature which would have produced death if suddenly applied. It is
almost impossible to overstate the importance of this faculty in enabling
organisms to survive changing conditions. Stimuli, then, may be said to
be powerful in proportion to their suddenness and intensity.

21. Assimilation. The process of changing food sub-
stances into protoplasm is called assimilation. It can be
effected only by protoplasm. Such foods may be relatively
simple substances or may be the complex protoplasm of other
organisms. The protoplasm of the green leaves of plants has
the power of utilizing the simple inorganic compounds, as

PROTOPLASM. 13

oxygen, water, and carbon dioxide, in a larger measure than
that of animals, which must have complex organic foods.

22. Growth and Reproduction. The result of assimila-
tion is the addition of new molecules of complex organic
matter among the molecules of the old. This produces growth.
It is to be denned as increase in mass. If this continues in-
definitely in excess of whatever may tend to destroy the pro-
toplasm, the increase in size may lead to the division of the
protoplasm. The parts may separate and lead an independent
existence. Such is reproduction. In its simplest form it is
merely growth beyond the limits of the individual. The cell
cannot continue to grow indefinitely. Its size is limited by the
necessity of physical support on the part of the soft protoplasm,
and by the relation between the outer surface, through which
the food must be taken, and the volume, which represents the
mass to be fed. The surface increases as the square of the
diameter, whereas the volume increases as the cube of the

FIG. i.

-p

FIG. i. Streaming of Protoplasm in the Amoeba. The forward motion of the
granules takes place more rapidly in the centre of the pseudopodium (/>). Those at
the margin fall behind those in the centre as the pseudopodium advances.

Questions on the figure. Why may the amoeba readily change its
form? Do its internal parts preserve a constant relation to each other?

diameter. It is apparent that the nourishing surface does
not increase as rapidly as the mass to be nourished, and in con-
sequence the time will come when the nourishment possible to
be absorbed will just nourish the volume, and growth must
cease. This condition may constitute an internal stimulus to

14 ZOOLOGY.

division. At any rate division furnishes a way out of the
dilemma and allows a renewal of growth of the daughter units.
23. Contractility. A body of living protoplasm seems
always to possess the ability to change its form in greater or
less degree. This results in motion of parts or of the whole,
and is called contractility. Movement or contractility is closely
related to irritability, and results from the action of stimuli,

FIG. 2.

s

FIG. 2. The circulation of protoplasm (p) in a cell of a stamen-hair of Tradescantia.
In the channels the granules move back and forth to the various parts of the cell.
The remainder of the cell is filled with cell-sap (j) which in these cells is colored.

Questions on the figure. In what respects are the activities of the
protoplasm necessarily limited in this cell as compared with the condition
in Amoeba? Why is circulation an appropriate term?

external and internal, upon the complex protoplasm. It is
made possible by the assimilation of food substances. These,
in being broken down, furnish the energy shown in motion.
The nature of .the motion resulting from contraction differs
somewhat, depending upon whether the protoplasm is en-

PROTOPLASM. 15

^eloped by a cell-wall or is naked. If without a wall, it may
send out foot-like projections into which there passes a stream
of granules, as in the Amoeba (see Fig. i) ; if enclosed, the
protoplasmic mass may rotate within the cell wall, or there
may be narrow channels in which the currents move between
banks of more stationary material. The latter motion is de-
scribed as circulation. (Fig. 2.)

24. Demonstrations. The teacher should, if possible, demonstrate
protoplasmic motion to the students with a compound microscope of good
magnification. The Amoeba will serve to illustrate the naked streaming
motion ; Paramecium, rotation ; the hairs from the stamens of Tradescan-
'tia beautifully illustrate circulation. (There is a cultivated species which
may be kept blooming in greenhouses at all seasons of the year.) Ciliary
motion may be shown in several of the large Protozoa, or by living cells
scraped from the oesophagus of the frog.

25. Dissimilation. Motion and the other responses which
protoplasm makes to stimuli necessarily represent chemical or
physical changes, or both, in the protoplasm. It is well known
that complex chemical substances, such as are found in proto-
plasm, can be made to yield energy when they are torn down
into simpler ones by some element which has an affinity for
some of the elements constituting the substance. The result
of this action is, always, simpler and more stable compounds
than the original, and therefore of less use in the further
freeing of energy. This tearing-down process is the opposite
of assimilation and is sometimes called dissimilation or katab-
olism. Oxygen is one of the most important agents in nature
for the freeing of energy by breaking down the complex
chemical substances. It unites with the carbon particularly,
and this union is one of the principal sources of energy which
animals show. The process is called oxidation and is essen-
tially the same thing that occurs when wood or coal is burned.
The energy belonging to the wood by virtue of its chemical
constitution is partly freed by the action of the oxygen in
uniting with the carbon and hydrogen, reducing the wood to
ashes, water, and carbon dioxid. In the stove the principal
form of energy secured is heat; but in appropriate engines,

1 6 ZOOLOGY.

locomotion and other forms of mechanical work, or light, or
electrical energy may be secured by the oxidation. So in
protoplasm, various types of energy may result from the tear-
ing down of the complex substances. Among these are animal
heat, motion, nervous energy and electrical energy.

26. Secretion and Excretion. As a result of the constructive and
destructive work already mentioned as characteristic of protoplasm cer-
tain substances, not themselves protoplasm, may be produced. If these
products are of further use in the animal economy, they are usually
described as secretions; if they represent the final reduction in the process
of tearing down, they are called excretions. Such materials may be de-
posited either within the protoplasm or at its surface. In the latter case
it may be deposited in a uniform sheet and produce a protective mem-
brane (cell wall). The presence of such a covering to the protoplasm
very materially modifies all the elementary activities which have been
described.

27. Demonstrations. The teacher should make microscopic demon-
strations of secretions and excretions : as starch grains formed in the
leaves of plants ; fat in adipose tissue ; cell-walls in plants ; crystals in plant
cells (see Botanies) ; intercellular substance in cartilage or bone. ,

28. Supplementary Topics for Library Work. Find and examine
some of the classic definitions of life. Examine more completely the
theories of protoplasmic architecture. In what ways would the presence
of the cell-wall bring about modifications of the protoplasmic activities?
Give an account of experiments showing the effect of some of the more
important stimuli on protoplasm (as light, heat, electricity). What of the
external conditions are so important as to merit the term "primary con-
ditions of life " ? Why may protoplasm be described as chemically un-
stable? Compare oxidation in the protoplasm with oxidation in ordinary
combustion.

29. Summary. i. Scientists are not agreed whether life
is merely the action of the ordinary chemical and physical
forces in connection with a peculiar substance, or represents
these, guided by a type of energy of a higher order.

2. Protoplasm, a chemical compound of exceeding complex-
ity and instability, is the " physical basis of life." Differences
in various living things are probably due to differences in the
chemical and physical structure of the protoplasm of which
they are composed.

3. Owing to the unstable character of the protoplasm it is
readily acted upon and changed by external forces; and the

PROTOPLASM. 17

various parts of the protoplasm act on each other in such a
way as to produce a display of energy. The agents are called
stimuli. Protoplasm responds to stimuli because of its irri-
tability and contractility. These latter powers belong natively
to protoplasm because of its physical and chemical composition.

4. Protoplasmic matter and the materials which are de-
stroyed in the production of energy are alike produced by the
assimilation of food substances into new protoplasm. This
is a most fundamental quality.

5. Growth is increase of mass, following the formation of
new substance by assimilation. The mere absorption of water
also results in growth. Growth leads naturally to reproduc-
tion.

6. Oxygen is one of the chief agents by which the unstable
compounds in the protoplasm are made to release their energy.
The breaking down of these compounds leaves unused mate-
rials which must be excreted. Respiration, which is a term
applied to the using of oxygen and the elimination of carbon
dioxid, and excretion are thus seen to be protoplasmic func-
tions immediately connected with its activity.

CHAPTER III.
THE ANIMAL CELL; ITS MORPHOLOGY AND PHYSIOLOGY.

30. Introduction. In studying the structure of organisms
two methods are open to the students of to-day. He may
begin with the whole adult individual and by dissection he may
reach a knowledge of the constituent parts, organs, tissues,
cells. This, the analytic method, is the method of history and
has given us the mass of details which we have at present.
On the other hand, it is possible to avail one's self of the
results of such studies, to assume the unit of structure which
is uniformly found, and, by a synthetic process, follow the
building up of an organism from its elementary parts. This
is the process which the development of the individual illus-
trates. It has the special advantage of emphasizing the funda-
mental unity of origin of the organs, and the likenesses of
organisms, and gives the true significance of differentiation
and development.

31. The Cell. Having discussed in Chapter II the sub-
stance in connection with which life manifests itself, it is
necessary to recall the fact that the protoplasm of an organism,
while connected in various ways, is separated by boundaries
into unit-masses, each mass having the essential qualities of
the whole. Each unit mass of protoplasm is called a cell. The
cell is not to be considered as the ultimate unit of structure;
it is itself a group of bodies which are in turn composite. It
is thus to be looked upon as an organised structure.

32. Cell Form. Cells, unhampered in the direction of
growth, tend to assume a spherical form. Agencies both in-
ternal and external, as nutritive processes, tension, pressure,
etc., may modify this in such a way that almost any form
may be found: polygonal, flattened, elongated, fibrous,

branched, etc.

18

THE ANIMAL CELL. 19

33. Size. While ordinary tissue cells are minute, there is
great variation in the size of cells. Many single-celled in-
dividuals are visible to the naked eye and egg-cells may be
several centimetres in diameter; yet many tissue cells are less
than .005 millimetre in diameter. Cells may be very much
extended in one or more directions. The outgrowths of nerve
cells for example may attain a length of several feet, as when
the nerve fibers extend from the trunk to the tips of the toes.

34. Structure. The following parts are to be distinguished
in the typical cell : ( i ) a general cell substance, partly liv-

FIG. 3.

FIG. 4.

FIG. 3. Diagram showing the principal parts of the cell and something of the
protoplasmic architecture as it might appear while living, a, alveoli or spheres in the
foam-work (see 17); c, centrosome; cy, cytoplasmic meshwork, containing granules;
nu., nucleus; n, nuckolus; v, vacuole; w, cell wall.

FIG. 4. Diagram showing principal parts of the cell as it appears when killed
and stained. The protoplasm shows more of a meshwork (cy), the spaces represent-
ing the alveoli, f, formed substance in alveoli. Other letters as in Fig. 3.

Questions on figures 3 and 4. If these cells are in reality 25 M in
diameter, how much are they enlarged in the drawing? (A* is .001 mm.).
Identify the various structures referred to in section 34.

ing protoplasm, partly non-living matter both organic and
inorganic; (2) usually a single highly differentiated nucleus
which contains living protoplasm and is clearly demarcated
from the substance about it; (3) one or more specialized
bodies known as centrosomes', (4) a cell wall or membrane
(Figs. 3 and 4).
The cell-substance or cytoplasm embraces that portion of

20 ZOOLOGY.

the living protoplasm (plasma} outside the nucleus, and the
more fluid cell-sap (chylema) which includes such non-living
materials as starch, fats, and inorganic matter dissolved in
water.

35. The Nucleus. The usually single nucleus lies im-
bedded in the cytoplasm and is ordinarily separated from it
by a thin membrane. Nuclei vary greatly in shape, size, and
degree of differentiation. While it is not always possible to
find definite nuclei in all cells, it seems probable that all cells
have nuclear material in one form or another at some stage
of their history. The internal structure of the nucleus is
equally as complex as that of the cytoplasm, having both liv-
ing and non-living portions. It usually consists of a network
qf threads (chromatin} readily stained by certain dyes. In
the meshes of this a less easily stainable material occurs
(achromatin) , a portion at least of which is living. One or
more deeply stainable bodies, called nucleoli, usually occur,
the real character of which is difficult to estimate.

36. Centrosomes or Centrospheres. These bodies lie in
the cytoplasm but are closely related to the nucleus, and ap-
pear to have an important place in certain phases of cell act-
ivity (see "cell division," 40).

At such times the cytoplasmic elements radiate from the
centrosomes in a very characteristic way (Fig. 7, c). The
influence extends into the nucleus and is accompanied by a
rearrangement of the chromatic elements. The origin of the
centrosomes is still a matter of disagreement. They are often
spoken of as attraction spheres from the fact that they ap-
pear to exert an attractive influence upon certain portions of
the protoplasm.

37. Cell- wall. A cell membrane usually surrounds the
protoplasm. It may be a non-living organic secretion, or may
consist of metamorphosed or altered protoplasm in connection
with such secretion. The wall is protective and supportive
in function, and varies much in thickness, resistance, etc.

THE ANIMAL CELL.

21

Animal cells as a rule are not provided with such well de-
veloped and resistant walls as are plant cells.

38. Cell Functions. Since the cell is only a definite mass
of protoplasm, its functions are in general those which have
already been described as protoplasmic functions. They are
merely localized within the cell. The cell wall when pres-
ent would naturally modify and limit in important ways,
the more active protoplasmic functions. In such cases the
independent motion characteristic of so many cells must be
accomplished by special devices. These frequently take the
form of cilia or flagella, which are thin protoplasmic projec-
tions used after the manner of oars. Locomotion of cells
is not confined to single-celled organisms, but is found in
many cells of the higher animals and plants as colorless

FIG. 5.

C

m.

FIG. 5. Modes of cell reproduction. A, B, and C, stages in the reproduction of the
Infusorian, Colpoda, by the breaking up of the protoplasm to form numerous cells.
A, encysted stage; B, protoplasm escaping, spores partly formed; C, spores com-
pletely separated (adapted from Rhumbler) ; D, budding in Chlamydomyxa, a lowly
Rhizopod. b, bud; cw., cell wall; m, mother cell; n, nuclear matter; s, spores.

Questions on the figure. Compare the process and the results of the
two modes of cell reproduction shown in this figure. Can you describe the
fate of the " mother " cell in the two cases ?

22

ZOOLOGY.

blood cells, sexual cells, etc., which have a distinct motion of
their own. The muscle cells of higher animals possess the
power of contraction and motion in a high degree.

39. Reproduction. The cell grows as a result of the nu-
tritive processes and reaches the limits of size determined by
its special conditions. The internal and external conditions
constitute a stimulus to the breaking up or division of the
protoplasmic unit. This may occur (i) by the irregular
breaking up of the protoplasm into numerous masses, each
of which has the essential qualities of the whole (Fig. 5, A and
B) ; (2) by budding, in which a process or several processes
appear on the cell, develop into bodies like the original cell, and
finally become separate from it (Fig. 5, D} ; (3) by division,
in which there is a division of the original protoplasm into
two essentially equal parts. In this case neither of the cells
can be considered the parent of the other.

40. Cell Division. Cell division may be effected in either
of two ways, (a) by direct or amitotic division, in which the

FIG. 6. Direct cell division (Amoeba). A, active specimen with pseudopodia; B, be-
coming spherical preliminary to division; C, beginning of elongation and constriction;
D, later stage; E, daughter cells forming pseudopodia. ec, clear ectoplasm; en, granu-
lar endoplasm; /, food vacuole; n, nucleus; ps, pseudopodium; v, pulsating vacuole.

Questions on the figure. Why is this properly called direct division?
What structures are divided? Are the resulting halves exactly or merely
roughly equal, apparently? Do you see any possible gain to the organism /
in such a division as this?

THE ANIMAL CELL. 23

nucleus and cell merely constrict into two nearly equal parts
(Fig 1 . 6) ; and (&) indirect or mitotic division. The latter is
the usual method and is very complicated. By means of it a
very even division of the substances and structures of the
nucleus, especially, seems to be secured.

The more striking stages in the process as it usually occurs are out-
lined in the text and figures which follow. The nucleus will be seen to
be especially active

1. In the quiescent or resting stage the structural elements are dis-
tributed in the way characteristic of the particular cell under examina-
tion (Fig. 7, A).

2. When division is about to take place, the chromatin elements in the
network of the nucleus assume the appearance of a coil 'or tangle of
thread (Fig. 7, 5). The nuclear membrane often disappears at this time.

3. The centrosome divides and the halves migrate to opposite poles of
the nucleus, and from them as centres radiations pass into the cell body
in all directions. Across the nucleus, from one centrosphere to the other,
thread-like*lines extend, producing the appearance of a spindle (Fig. 7, C,
sp). In the meantime the coil of chromatin has been unraveled and has
broken up into a definite number of pieces (chromosomes) which often
form into V-shaped loops. After certain evolutions, under the influence
of the centrospheres apparently, these loops come to lie in the equatorial
plane of the spindle, the apices of the loops pointing toward the centre of
the nucleus. This is called the astroid stage (Fig. 7, C). The process up
to this point is known as the prophasc or preparation stages.

4. Each of the chromatin loops next splits longitudinally into two.
This is the metaphase or middle stage (Fig. 7, D).

5. Each of these halves now begins to move toward its appropriate
pole or centrosome (Fig. 7, E). As these half-loops leave the equator and
collect about the poles they give rise to a double-star appearance or dias-
troid stage (Fig. 7, F). This is the anaphase.

6. The loops of chromatin collected at each pole are reconstructed into
a coil which then passes into the resting stage at the new position, a
membrane is formed, and the daughter nucleus is complete. The nuclear
spindle disappears, the radial appearance about the centrosomes, and even
the centrosome itself, may disappear or become inconspicuous.

7. Accompanying or following the last nuclear changes the cytoplasm
may have become constricted into two masses, or separated by the formation
of a wall perpendicular to the axis of the spindle (Fig. 7, G, H). The
daughter cells may separate or remain united. These final stages are known
as the telophase. Cell division is at the beginning of all the complexities
of structure found in the higher forms of animals. Each sexually produced
organism commences life as a single cell, from which the adult is formed
by cell-division, and the clinging together of the daughter cells.

2 4

ZOOLOGY.
FIG. 7.

: "v:-C S P . S ,-^jfej

''ffil;-' sp - 'i- N

IMG. 7. Indirect or mitotic division (diagrammatic); A, resting mother nucleus; B,
toil stage, with the centrosomes separating; C, D (metaphase), and E, stages in the divi-
sion of the chromosomes; F, diastroid (anaphase) stage; G and H show the return
of the daughter nuclei to the coil and to the resting condition, and division of the
cytoplasm, and the formation of the dividing wall: c, centrospheres; cl, chromatin
coil; chr, chromosomes; nu,, nucleus; n, nucleolus; sp, nuclear spindle; w, cell wall.

Questions on the figure. What structures possessed by the original
cell are divided in this process? In what order? Why is this termed
"indirect" division? Which is the more common, the direct or the
indirect? Can you see any special gain secured by this method? Describe
the behavior of the nucleolus and the nuclear membrane by comparing tAis
with other figures in reference books.

THE ANIMAL CELL. 25

41. Functions of the Nucleus and Centrosomes. While we can fol-
low some of the externals of the various cell activities, the manner of
their occurrence and their causes are in the greatest obscurity. We are
not able to say just what part is performed by the different structures
involved. It is hazardous to say that one structure is more important
than another; yet it seeems to be proven that the nucleus is quite essential
in cells which possess nuclei, for the proper performance of even the ordi-
nary nutritive functions. Some of the unicellular animals may be* artifi-
cially mutilated in such a way that the lost parts may be regenerated and
the normal form restored. A relatively small piece of the Protozoan,
Stentor, for example, can reproduce the whole, if a portion of the nucleus
be present. A much larger piece without nuclear material is wholly un-
able to regenerate lost parts, and even seems unable to control or exer-
cise the ordinary assimilative functions. The phenomena of indirect cell
division show that activity on the part of the centrosomes and nucleus
precedes that of the cytoplasm. Experiments also show that the division
of the cytoplasm may be checked or interrupted by external influences
without interfering with the division of the nucleus. On the other hand
nuclei separated from cytoplasm are incapable of continuing their func-
tions. We are at least safe in saying that these three bodies, the centre-
some, the nucleus, and the cytoplasm act as intracellular stimuli upon each
other, and that all are important in the work of 'the cell.

42. Exercises for Library and Laboratory. The teacher should
secure preparations of properly stained cells showing the principal struc-
tures; also if possible some of the stages of cell division (see Appendix;
suggestions to teachers).

What are chromosomes? In what respects and to' what extent do
nuclei differ? What is meant by the "cell-doctrine"? Give an outline
of its history. Compare, the various series of figures in your library illus-
trating the stages of cell -division.

43. Summary.

1. The cell may be considered as the unit of structure, and
is to be denned as a "nucleated mass of protoplasm with or
without a cell membrane."

2. The cell may also be considered the unit of function, in
the sense that it embodies all vital functions in epitome.

3. The structure of the typical cell may be outlined as fol-
lows:

(a) Cell body

Cytoplasm living.
Cytolymph non-living, fluid.
Metaplasm non-living, solid.

26 ZOOLOGY.

(&) Nucleus:

Nucleoplasm living.
Chromatin.
Achromatin.

Nucleolymph non-living, fluid.
Metaplasm non-living, solid.
[Protoplasm = Cytoplasm -j- nucleoplasm.]

(c) Centrosome.

(d) Cell wall.

4. In addition to the general functions of protoplasm which
cells possess we need to consider in connection with cells the
additional functions :

(a) Locomotion.
(&) Reproduction.

5. Reproduction of cells occurs by fragmentation, by bud-
ding, and by division. Division may be either direct or in-
direct.

6. The following diagram, adapted from Flemming will
serve to represent the stages in indirect division :

One mother nucleus. Two daughter nuclei,

(a) Resting stage. Resting stage (

(6) Coil stage i Coil stage (f.

}=

(<r) Astroid stage, f n/^... Astroid stage (e. = anaphase.

V Prophase.
(d) Division of chromatin loops = we taphase (d. /

7. The important effect of this complicated process is, ap-
parently, to secure an equal division of the nuclear elements
for the daughter cells. The cytoplasmic. elements in the
daughter cells may be strikingly unequal.

8. The exact functions of the various structures in the cell
are not known. They cannot be understood until the chem-
ical and physical nature of living protoplasm is known. The
cytoplasm, the centrosomes, and the nucleus seem to act as
stimuli to one another, in assimilation, growth, and division.

J

CHAPTER IV.
FROM THE SIMPLE CELL TO THE COMPLEX ANIMAL.

44. The Individual as a Cell-composite. In the simplest
animals, as the Protozoa, the individual consists of a single
cell, and the life history of the individual animal is such as
has already been seen to belong to the cell (Chapter III). In
such an individual one cannot speak of organs in the ordinary
sense, for organs as we shall see are made up of cells bound
together in the doing of certain work. Yet it is important to
remember that there are none of the necessary duties of life,
such as getting food, digesting it, breathing, moving, repro-
ducing, and the like, which are not well done by these simple
one-celled animals. The many-celled animals agree with the
simpler ones in that they too start life as single cells ap-
parently quite as simple as the one-celled animals themselves.
When the cells divide, however, the daughter cells do not
separate as in the Protozoa, but form a mass of cells by cling-
ing together. Owing both to internal and external forces
the cells in the mass do not long remain alike, but soon show
such differences among themselves as serve as the basis for
the great variety of structures found in the bodies of the
higher animals. The change from the simple cell to the com-
plex condition in the adult animals is not a sudden one,
but takes place very gradually and the work which was
formerly done by the single cell is divided up among the
groups of different cells composing the body. The division
of the work to be done makes possible and necessary the spe-
cializing of certain cells to do each part of it, and the differen-
tiation of structures makes it possible to do each separate task
better than before. Thus division of labor and differentiation
of parts go hand in hand as we pass from the simple to the
complex animals.

27

28

ZOOLOGY.

45. The Fertilized Ovum the Starting Point. In speak-
ing of the development of the adult animal from the simpler
condition of the single cell it is necessary to remember that
this cell, which has the power of giving rise to a complex in-
dividual and is called a fertilized ovum, has a history that is
very important. The fertilized ovum represents the union of
two distinct cells, known as germ or sexual cells, which are
ordinarily quite different in appearance and produced by dif-
ferent kinds of individuals, males and females. Both classes
of cells may be produced by the same individual. This union
does not produce a double cell, but the parts of each seem to
fuse with those of the other in a very complete way.

46. The Ovum. The female germ cell is known as the
ovum, and is typically a spherical cell with abundant nourish-

FIG. 8. Types of ova. A, primitive amceboid ovum of Sponge; B, semi-diagram-
matic figure of spherical ovum of Sea-urchin in which the yolk is uniformly distributed;
C, figure of a spherical ovum (such as may be found in some Worms or in the Frog) in
which the yoke tends to collect at one pole, p.p., and the nucleus and protoplasm at the
other, a.p.; m, micropyle; mi, germinal vesicle (nucleus); n, germinal spot (nucleolus) ;
y, yolk spheres.

Questions on the figure. What are the points of agreement in these
three ova? The chief points of contrast? What is the function of the
micropyle? Is a micropyle always present in ova? Why are the poles of
the ovum appropriately called active and passive?

ment and inactive as compared with the male cell. It often
has an especially well-developed cell-coverii%-. Its nucleus is
sometimes called the germinal vesicle and its nucleolus, the
germinal spot (Fig. 8). The ovum must be distinguished

FROM SIMPLE CELL TO COMPLEX ANIMAL. 29

from what is popularly known as an egg. The latter term
is loosely used to describe the fertilized ovum more or less
developed, together with its nutritive and protective coats
such as occur around the eggs of birds and reptiles. Ova
differ very greatly in size. The largest are found among
the birds. The " yellow " of these eggs represents the real
size of the ovum. Variations in size are due not so much to
a difference in the amount of protoplasm as to a varying
amount of food or yolk in the cell. The food may be uni-
formly distributed throughout the ovum, mingled with the
protoplasm, or it may collect at one pole, forcing the active
protoplasm to occupy the other pole (Fig. 8, C). The yolk
furnishes food to the young individual or embryo in its early
development.

47. The Spermatozoon or male element is ordinarily in
striking contrast to the female. It is typically very small,

FIG. 9.

A

FIG. 9. Types of spermatozoa. A, from the round worm (Ascaris) with a cap,
somewhat amoeboid; B, from the Crayfish, with numerous projections; C, from Frog; D,
from Sea-urchin, h, head; m, middle piece; n, nucleus; t, tail or flagellum.

Questions on the figure. What are the chief points of similarity and
dissimilarity in these spermatozoa? How do they agree with, and how
differ from, the ova in Fig. 8? How do they differ from the average cell?
What parts of the structure of typical cells are believed to be represented
in the sperm cells?

3 ZOOLOGY.

active, and with thin protoplasmic projections (Fig. 9).
Structurally, the typical spermatozoon consists of a " head "
piece, a middle piece, and a " tail " or flagellum. The head
is composed chiefly of the chromatic material of the nucleus.
A delicate covering of cytoplasm envelopes the head and is
drawn out into the projection known as the tail (Fig. 9, D).
The middle piece has been variously interpreted. Some re-
gard it as the achromatic part of the nucleus, others as con-
taining the centrosome of the male cell.

48. Maturation of the Ovum. After the egg cell has been
produced by the maternal germinative tissue, and before the
union of the male and female cells, the nucleus of the egg-
cell approaches the surface of the cell and divides twice in
close succession by the indirect or mitotic method (see Fig.
10). The cytoplasm of the ovum does not divide equally.
A small amount of cytoplasm enclosing a half of the nuclear
material forms a bud-like cell on the surface of the ovum dur-
ing each of the two acts of nuclear division. These minute
cells, the polar bodies, are cast off from the egg and perish.
They are to be regarded as abortive or vestigial eggs. In the
egg-nucleus there remains therefore only one-fourth of the
original nuclear material. The remnant returns to the cen-
ter of the cell and is termed the female pronucleus. It con-
tains only one-half the number of chromosomes found origin-
ally in the ovum. This elimination of the nuclear material
including the reduction of the chromosomes is known as the
maturation or ripening of the ovum. The abortive cells or
polar bodies are not known to have any function other than'
this elimination of material. The nuclei of the sperm cells
during their formation undergo a similar reduction of the
chromosomes but in a somewhat different way. In these are
no abortive cells. All the daughter cells form sperm.

49. Fertilization. The union of a sperm cell with the
ovum constitutes the act of fertilization. Often there is a
special aperture (micro pyle*) in the outer egg-membrane

FROM SIMPLE CELL TO COMPLEX ANIMAL. 31

through which the spermatozoon finds entrance. Usually only
one sperm cell gains admission to the interior of the ovum,
whether by way of the micropyle or through the unmodified
membrane. Changes normally occur in the membrane as soon
as one sperm enters, by which all others are excluded. In

FIG. 10.

FIG. 10. Four stages in the maturation and fertilization of the ovum (partly dia-
grammatic). A, formation of the, second polar body and the entrance of the sperma-
tozoon; B, the male and female pronuclei, the former with aster about the centrosome;
C, nuclei coming together; sperm centrosome has formed two; D, pronuclei uniting to
form segmentation nucleus; asters producing spindle preparatory to cleavage, e.n., egg
nucleus; p.b., polar bodies; s, spermatozoon; s.c., sperm centrosome and aster; s.n.,
sperm nucleus; se, segmentation nucleus produced by the union.

Questions on the figure. In what respect is the formation of the polar
bodies similar to ordinary indirect cell-division? In what respects different
from it? Is there any difference shown in the figures between the first
and the second polar bodies? In what, apparently, does maturation con-
sist? In what way does fertilization appear to compensate for the loss in
the formation of the polar bodies?

eggs which have been kept too long or subjected to unfavor-
able conditions, the response of the membrane may not be so
quickly effected and multiple fertilization may occur. Such
fertilizations may produce monstrosities. The sperm nucleus
is now called the male pronudens. It migrates toward the

3 2 ZOOLOGY.

female pronucleus and fuses with it; thus is formed the first
segmentation nucleus. With the addition of the chromosomes
in the male nucleus the fertilized ovum contains the same
number of chromosomes as before maturation, which in each
species of animals is a constant number. It appears that fer-
tilization restores to the female cell essentially what it lost in
the process of maturation, and in addition stimulates it to
active nuclear and cytoplasmic division as indicated in the next
paragraph. Follow the process in Fig. 10.

50. Segmentation or Cleavage. Following shortly upon
fertilization, if conditions are favorable, ordinary mitotic
nuclear division begins and the ovum divides promptly into
2, 4, 8, 16, etc., cells (blastomeres} . The resulting cells be-
come smaller and smaller with each division, since the whole
egg-mass does not increase appreciably in size meanwhile.
The first three cleavage planes are usually perpendicular to
each other. Their position is much modified, however, by
the presence of food or yolk substance in the egg. The yolk
in general retards cleavage. If the yolk is in small quantity
and is uniformly distributed through the egg, the blastomeres
will be about equal in size (Fig. n, A}, and will continue to
divide with practically equal promptness. If there is much
of the yolk it is not likely to be uniformly distributed. Under
the influence of gravity and internal forces, the yolk is likely to
collect at the lower, and the protoplasm and nucleus at the up-
per pole of the ovum (Fig. n, B, C}. The protoplasmic pole
is known as the active or formative pole and the lower, as the
passive or nutritive pole. The polar bodies are normally freed
at the formative pole. Under these circumstances the blasto-
meres at the nutritive pole are larger and divide less rapidly
than those in which the protoplasm is in excess. If the
yolk is excessive in amount that portion" of the ovum in which
it collects may be totally prohibited from dividing.

51. Forms of Segmentation. The conditions suggested above give
rise to the following classes of segmentation.

FROM SIMPLE CELL TO COMPLEX ANIMAL.

FIG. ii.

33

FIG. ii. Cleavage and gastrulation (not drawn to scale). The vertical rows A, B,
C, and D represent different classes of ova. A, an ovum with little yolk; B, one with
considerable yolk collected at the lower pole (p.p.) ; C, one with a large amount of dense
yolk crowding the protoplasm to one side (a..) ; D, ovum with dense yolk collected
at centre. The numerals (1-4) indicate stages in cleavage and gastrulation: i, ova;
2, 4-8 celled stages of segmentation; 3, blastospheres, blastula stage; 4, gastrula stage.
a, archenteron; a. p., active pole; bl, blastoderm; bp., blastopore; ec, ectoderm; en, ento-
derm; ma., macrospheres; mi, microspheres; p.p., passive pole; s.c., segmentation
cavity; y, yolk; y.c., yolk cells.

Questions on the figure. -What constitutes the difference between the
active and the passive pole? Judging from the drawings and from your
references to texts . does gravity have any influence in determining the
position of these? Your evidences? Which pole gives rise to ectoderm?
Why does the food substance interfere with segmentation? What is the
difference between the segmentation cavity and the archenteron? How
does the presence of food substance modify the formation of an archen-
teron?

34 ZOOLOGY.

A. Total segmentation.

I. Equal : in which there is little yolk material, and that is well
distributed. (Illustrated in most of the lower invertebrates
and mammals.) Fig. n, A.

II. Unequal : in which there is a moderate amount of yolk which
accumulates at the passive pole. The cells at the active pole
are more numerous and smaller than at the passive. (Illus-
trated in many mollusks and in the amphibia.) Fig. n, B.

B. Partial segmentation.

I. Discoidal : in which there is an excessive amount of yolk, with
the nucleus and a small mass of protoplasm occupying a disc
at the active pole. This disc alone segments, and the embryo
lies upon the yolk. (Illustrated in the eggs of fishes, birds and
reptiles.) Fig. n, C.

II. Peripheral : in which an excess of yolk collects at the centre
of the ovum, with the protoplasm at the periphery. The divid-
ing nuclei assume a superficial position and surround the unseg-
mented yolk. (Illustrated in the eggs of insects and other
arthropods.) Fig. n, D.

52. Blastula and Morula. As cleavage continues the
blastomeres remain associated in a spherical mass. The in-
dividual cells project beyond the general surface not unlike
the lobes of a mulberry, and for this reason this stage is called
the morula or mulberry stage (Fig. n, <?). By the growth
of the cells and by the imbibition of water the morula may
become a hollow sphere of cells (blastula) the central cavity
of which is filled with fluid. The cavity is termed the segmen-
tation cavity (Fig. n, s.c).

53. Gastrula. In those eggs in which the segmentation is
total, a next important step is the pushing in of that side of
the blastula which corresponds to the original nutritive pole.
The process is known as invaginution, and the product as a
gastrula (Fig. n, 4). It takes place much as one might sup-
pose one side of a hollow rubber ball to be dimpled or infolded
by the exhaustion of the air within. The gastrula is to be de-
scribed as made up essentially of two layers of cells, one ex-
ternal and called ectoderm or epiblast, and one within called
entoderm or hypoblast (Fig. n, 4). The segmentation cavity
may be wholly obliterated ; in that case the entoderm and ecto-
derm come to lie in contact. The cavity of the invagination

FROM SIMPLE CELL TO COMPLEX ANIMAL.

35

of the gastrula is the archenteron or embryonic digestive
tract; the opening into it, that is, the mouth of the gastrula,
is the blastopore (Fig. n, bp). In morulas in which the seg-
mentation cavity is small and the cells at the nutritive pole are
large (Fig. n, C, 4) this simple condition is much obscured,
and invagination as described above becomes impossible. Nev-
ertheless early in development the cells which produce the two
primitive layers are to be distinguished, and their relations are
always substantially as detailed. If the term gastrula is ap-
plied to these we have to say that they are formed in some
other way than by ordinary invagination.

54. Library Reference. Let students report briefly on gastrulation by
overgrowth (epibole), and by delamination. Compare the results attained
by the various methods. Note what is constant in the methods and in the
results.

55. Germinal Layers. The ectoderm and entoderm have
thus far been mentioned as the primary germinal layers of
cells. Some of the Invertebrates have only these two layers,
but in most cases a third mass of cells comes to be situated
between the ectoderm and entoderm, from which important
organs are derived. The third or middle layer (mesoderm
or mesoblasf) differs somewhat in its origin in the different

FIG. 12. Modes of forming mesoderm (diagrams modified from Whitman and
Selenka). A and B, special mesoblasts distinguishable early in segmentation (Annelid):
A, surface view from active pole; B, sectional view of same, ec, micromeres destined
to form ectoderm; en, macromeres destined to form entoderm; m, primitive mesoblast
which produces the mesoderm. C, amoeboid mesodermal cells (c) budding from ento-
derm into the segmentation cavity (s.c.), in an Echinoderm. a, archenteron.

FIG. 13. Mesoderm formed by pouches from entoderm after, gastrulation. a, primi-
tive gut; bp., blastopore; cor, body cavity, formed from pockets of the archenteron; ec.,
ectoderm; en., entoderm; m., mesoderm; m.so, body-wall mesoderm; m.sp., visceral
mesoderm; s.c., segmentation cavity.

Questions on figures 12 and 13. Enumerate the three modes of meso-
derm formation figured here. In which type may the mesoderm be identi-
fied most early in the embryonic development? By comparing with other
texts determine in what groups of animals the mesoderm is formed as in
Fig. 13. What are the differences between A and B of Fig. 13?

groups of animals. It may originate (i) from the multipli-
cation of a few special cells which, before invagination, be-
come distinct from those that are to form ectoderm and ento-
derm (Fig. 12, A and B, m} ; (2) by means of isolated, wan-
dering cells budded from the other two layers, particularly the
entoderm (Fig. 12, C ' , c) ; or (3) from entoderm, in the form
of pouches or of solid buds of cells which arise from the walls
of the archenteron and extend into the segmentation cavity
(Fig. 13, m). In some instances there may occur a combina-
tion of these methods.

56. Coelom. When the mesoderm develops by the last
mentioned method, i. e. by the evagination of the wall of the
primitive gut (Fig. 13, m), we see a p%ir of 'folds, or a series of
pockets, the cavities of which are at first continuous with the
archenteron, but later become separate from it and entirely
surrounded by the mesodermic layers. The outer wall of the
mesodermic pouches joins the ectoderm and forms a body
wall, and the inner applies itself to the entodermal wall of the
gut. The space between is the ccelom or body cavity. When

FROM SIMPLE CELL TO COMPLEX ANIMAL. 37

the mesoderm arises as a solid mass, instead of a pocket, the
body cavity is formed by the splitting of the mass into an
inner and an outer portion. When the ccelom is formed by
several pockets the cavities of these may ultimately coalesce,
forming a single body cavity. Such a cavity is found in all
the vertebrates and in the higher invertebrates, although it
may become more or less obscured and modified in the adult.

57. Differentiation of Organs and Tissues. We have al-
ready in these three layers and their foldings the fundamental
outline of that differentiation which is to give us the com-
plex animal form found in the adult. From these layers,
singly or in combination, all the tissues and organs of the
body arise. The various layers become locally thickened,
folded; or otherwise modified in form by rapid cell division,
thus producing the beginnings of organs. At a later date
differentiation takes place among the cells, and tissues arise
(see next chapter). In general each layer gives rise to such
structures as its position and relation to the outer layers would
suggest. This is especially noticeable in the ectoderm and en-
toderm. The former is more closely related to the outside
world, and from it are produced the protective and sensory
structures. These include the outer portion of the skin and
the hard parts often associated with it, and the whole nervous
system together with the sensitive portions of the organs of
special sense. The entoderm is derived from the cells which
contain, or at least are closely related to, the food originally
stored in the ovum (Fig. n), and it comes to lie in the in-
terior of the embryo. It furnishes the lining of the adult
digestive tract as well as the essential parts of the glands aris-
ing from it. The mesoderm gives origin to the muscles and
to the supportive or skeletal structures generally. Many of
the organs are made up of contributions from two or all of
these germinal layers. Students must be referred to special
textbooks on embryology for a more extended account of the
manner in which the germinal layers give rise to adult organs.

38 ZOOLOGY.

58. Summary.

1. All the higher animals begin life as a single cell and
reach their adult condition by a continuous series of divisions.
By the growth and specialization of the cells arising from
these divisions the great complexity of the adult body is pro-
duced.

2. This initial cell the fertilized ovum represents the
fusion of two independent and unlike cells: the ovum (fe-
male) and the spermatozoon (male).

3. Before the union (fertilization) occurs, the ovum re-
duces its nuclear material, by two successive divisions, to one-
fourth its original amount and the chromosomes to one-half
their original number, without a corresponding reduction of
the cytoplasm. The spermatozoon in its development seems
to undergo a similar reduction of chromosomes.

4. After the union of the male and female cells the fertil-
ized ovum divides rapidly (segmentation or cleavage) form-
ing a mass of cohering cells. The nature of these cells and
of the mass depends much on the amount of yolk in the ovum
and on its distribution.

5. By processes which differ in different animals accord-
ing to the nature of the segmentation, the cells become ar-
ranged with a layer outside (ectoderm), a layer within (ento-
derm), and from these a third layer or mass of cells lying be-
tween the other two (mesoderm). The entoderm bounds a
cavity (archenteron) which communicates by a pore (blasto-
pore) w.ith the outside world. Within the mesoderm may
be found a cavity (ccelom).

6. The ectoderm gives rise to the outer portions of the
skin, its protective and sensory structures, to the nervous sys-
tem, and frequently to the lining of the openings into the
body. The entoderm lines the principal part of the digestive
tract. The mesoderm gives rise to most of the other struct-
ures of the body.

59. Suggestive Topics for Library Work.

i. What suggestions have been offered as to the advantage

FROM SIMPLE CELL TO COMPLEX ANIMAL. 39

of the addition of the male nucleus to that of the female in
fertilization? Has a similar result ever been attained arti-
ficially by means of chemical or other stimuli ?

2. What suggestions have been offered as to the signifi-
cance of the process of maturation? Trace the maturation
of the sperm cells more fully.

3. What classification of ova do the textbooks make?
What is the basis of the classification? To what extent do
eggs of different animals vary in size, shape, envelopes, etc.?
Give examples.

4. Is there any explanation of the fact that there is such
a difference in the amount of food substance in the eggs of
different animals?

5. Trace out by reference to a textbook of embryology the
principal changes by which the adult digestive tract is derived
from -the simple condition found in the gastrula (archente-
ron). What is the fate of the blastopore? How does the
permanent mouth originate?

60. Exercises for the Laboratory.

The teacher should secure demonstrations of some of the
smaller ova (as of the snail, fish, sea-urchin) for examina-
tion with the microscope. Compare the ovum taken from the
ovary of a hen with a new laid egg, noting especially the
structure of the latter. Obtain spermatozoa from the testis
of a recently killed animal (as mouse, fowl, etc.) and ex-
amine with highest powers of the microscope. If possible
secure permanent mounts of segmenting eggs of sea-urchins,
showing the 2, 4, 8-celled stages.

CHAPTER V.
CELLULAR DIFFERENTIATION. TISSUES.

61. Two things of importance happen as the organism de-
velops from the simple condition of the ovum to the great
complexity of structure in the adult: (i) the increase in the
number of cells, which is quantitative in nature, and (2) the
differentiation of cells, whereby the cells of the various parts
become very diverse in shape, composition, and powers. This
is a qualitative change. It is not yet fully known how much
of the difference in the cells of the various tissues is due to
qualitative differences in the daughter cells of a given division,
and how much is due to external influences and the interrela-
tions of the cells after division. We know that gravity act-
ing on the food substance of the ovum before division does
produce such differences among the daughter cells of the
early cleavage stages as lead to results as diverse as ectoderm
and entoderm. On the other hand, it has been shown by ex-
periment that, even as high up in the animal scale as the
lower vertebrates, the blastomeres of the two or four-celled
stage may be shaken apart and each develop into a small but
perfect embryo. This experiment shows that up to this stage
no specialization has taken place which limits the products
that come from these cells. The blastomeres do not so de-
velop after the 8 or i6-celled stage is reached, so far as is
known. We are ignorant of the causes which determine that
one cell shall develop into a muscle cell and its neighbor into
a bone cell.

62. Tissues. A tissue is to be defined as a group of similar
cells suited by their differentiation to the performance of a
definite function. This differentiation affects the size, shape,
and the interrelations of cells, and likewise the chemical and
physical structure of the protoplasm, in such a manner as to

40

CELLULAR DIFFERENTIATION. 4!

cause great variation in their powers and activities. The
chemical differences are especially shown in excretion and se-
cretion whereby various sorts of materials are deposited with-
in and between the cells of the different tissues. The ma-
terial deposited between the cells is known as intercellular sub-
stance. The intercellular substance differs much in character
and amount. Both the cells and the intercellular substance
are important in enabling the tissue to perform its work. In
general if the tissue is active (as muscle) the cellular differ-
entiation is the important point; if, however, the function is
a more passive one, as support or protection, the nature of
the intercellular substance rather than the cells determines its
character (bone, connective tissue).

63. Classification of Tissues. From a physiological point
of view tissues may be classed in one of two groups : vegeta-
tive, and active. The vegetative tissues are those which per-
form the more passive functions, as nutrition, protection, sup-
port, etc. They resemble the plant tissues in their functions.
The two chief classes of vegetative tissues are: epithelial or
bounding tissues, and supportive or connective tissues. The
active tissues may be looked upon as the characteristic tissues
of animals. The muscular and nervous tissues belong to this
group.

64. Epithelial Tissue. This tissue is characterized by its
primitive form, i. e., by its relative lack of differentiation, by
the fact that it is the first to appear in individual development
(ectoderm and entoderm in the gastrula), and by the absence
of intercellular substance. It is a bounding tissue and consists
typically of a single layer of cells, although several layers may
occur. Epithelium bounds, by its own cells or their products,
the outside of the body, the lumen of the digestive tract and
its outgrowths, as well as the body cavity and the structures
contained in it.

65. Kinds of Epithelial Tissue. Located in a position
superficial to the other tissues, epithelium is subject to a wide

4 2 ZOOLOGY.

range of variation both as to form and function. Besides its
primary work as a protective layer, the epithelium may have
a glandular function, being favorably situated for the final
elimination of products from the body. Since it is especially
exposed it is the layer best adapted by position to receive those
external stimuli which we know to play such an important role
in the life of all organisms. The position of the epithelium
also renders it specially liable to destruction. To compensate
for this its primitive or undifferentiated character makes it
particularly capable of regenerating portions of itself which
may have been lost. Epithelium is often especially active also
in the regeneration of other than simple epithelial structures.
In close connection with this latter regenerative quality is to
be considered the fact that epithelium gives rise to the re-
productive or sexual cells by which new individuals are pro-
duced. The foregoing enumeration of functions suggests the
physiological classification of epithelia : bounding, glandular,
sensory, and reproductive epithelia. The same layer may
fulfill several of these functions at once.

66. Bounding Epithelium. The ordinary protective epithelium may
be made up of cells cuboidal in shape (Fig. 14, B), or columnar (Fig. 14,
A), or much flattened (Fig. 14, C). In extreme cases of flattening and
hardening we have squamous epithelium, e. g. the outer cells of the human
epidermis. Motile protoplasmic projections often extend from the free
surface of the epithelium. Flagellate epithelium (Fig. 77, D) has one such
projection from each cell, whereas ciliate epithelium (Fig. 14, D, ) has
numerous small ones. Cilia are more common in the lower groups of
animals, but are found even in mammals, in the moist internal passages, as
in the nose, trachea, etc.

Membranes bounding the body cavity are called serous membrane
(endothelium} . The lining of the digestive tract is described as a mucous
membrane.

Epithelial cells often secrete upon their outer surface a layer of mate-
rial (cuticula), which serves to protect the cells beneath and the organism
as a whole from external influences (as the covering of the cray-fish).
From the epithelium arise various outgrowths, as scales, hair, feathers,
and the like.

67. Glandular Epithelium. The ordinary columnar or pavement epi-
thelium may here and there present cells or areas of cells which are spe-
cially active in producing and pouring out on their free surface certain

CELLULAR DIFFERENTIATION.

43

materials, called secretions. In its simplest form the gland or secreting
surface may consist of a single cell, as the goblet or slime cells (Fig. 15,0).

FIG. 14.

II

1

i

i

It

,cu,

FIG. 14. Various kinds of epithelial cells (semi-diagrammatic). A, columnar; B,
cuboidal; C, pavement; D and E, ciliate (sectional views). In F is shown the surface
view of pavement epithelium, cl., cilia; en., cuticula.

Questions on the figure. For what different uses would you judge
these variously shaped epithelial cells to be suited? Under what circum-
stances and on what surfaces would you expect to find each type? Com-
pare with your reference texts and see if they are so found. Under what
circumstances is a cuticula to be expected? Where would it be a disad-
vantage? What are cilia?

FIG. 15.

,-a.

FIG. 15. Glandular Epithelium, a, goblet or slime cells, unicellular glands; b,
similar cells which have become depressed below the surface, and empty their secretion
through a duct.

Questions on the figure. Are the glandular cells modified epithelial
cells ? In what respects do they differ from the cells about them ?

44

ZOOLOGY.

Such a cell may become much enlarged and sink below the general
level -of the epithelium, retaining in the meantime a narrow connection
with the exterior (Fig. 15, &). Multicellular glands represent areas of such
cells which have sunk below the surrounding surface, forming a tube- or
flask-shaped cavity, which may become very much branched. Glands with
such branched ducts are described as compound. They consist of numer-
ous final secretory sacs communicating by ductules with a common duct
or outlet to the surface. Transitional conditions between the simple
secretory epithelium and the compound gland may be seen in Fig. 16.

68. Sensory Epithelium. In the lower animals there may be found
here and there over the surface of the body modified epithelial cells, which

FIG. 1 6.

D

C

FIG. 16. A series of diagrams showing progressive stages in the development of a
multicellular gland from an area of glandular epithelial cells. C and D show two some-
what different types of gland, the cup-shaped and the tubular, e, bounding epithe-
lium; g, gland cells; d, duct; c, connective tissue.

Questions on the figure. How do the compound glands seem to arise
from the simpler condition? What is the evidence that glands are lined
throughout with epithelium? What is gained in the sinking of the glands
below the surface?

are specially capable of being stimulated by contact or other stimuli to
which the organism may be exposed. Likewise in higher forms we find
highly specialized areas of sensitive cells, which can be shown to belong
primarily to the epithelium. These are the end organs of special sense,
as touch, sight, and the like, and they get their special value from their

CELLULAR DIFFERENTIATION.

45

connection with what will be described presently as the nervous tissues of
the central nervous system. The sensory cells are typically thread-like

FIG. 17. Sensory and muscular epithelium. A, sensory epithelium, from Worm,
showing some of the epithelial cells (e) modified into sensory cells (s). B, epithelial
cells from Hydra showing contractile or muscular processes at base (n).

Questions on the figure. Is there anything to suggest that the sen-
sory cells are modified epithelial cells? What are the principal changes
which they have undergone as compared with the unmodified epithelium?

FIG. 18.

FIG. 1 8. Diagram of a portion of the ovary of Sea-urchin showing the eggs arising
from the epithelium (reproductive epithelium) by constriction, e, epithelium; o, ova
in different stages of growth.

Questions on the figure. What is an ovary in its simplest form? Is
the reproductive epithelium ectodermal, entodermal, or mesodermal in
origin, as a rule?

or hair-like in form, often extended as fine fibres at the inner end, whereby
connection is established with the nerves (Fig. 17, A).

4 6

ZOOLOGY.

69. Reproductive Epithelium. The sexual cells, both male and female,
are developed from epithelium, -ectodermal, entodermal, or, as is usually
the case, mesodermal. The budding of the sexual epithelium, in the devel-
opment of the germ cells suggests the formation of glands (Figs. 18, 19).
The sexual cells often develop at the expense of the epithelial cells about
them.

FIG. 19.

FIG. 19. Section through ovary of a young Mammal (modified from Wieder-
sheim). The eggs (o) are seen to be formed from the epithelium by a process- some-
what more complex than in Fig. 18. c, connective tissue of ovary; e, epithelium; f,
follicle of epithelial cells in which the ova ripen; o, ova in different stages of
ripeness.

Questions on the figure. In the ovary of the mammal what addi-
tional service does the epithelial layer render the ovum after its forma-
tion? Is it apparent that there is anything gained by the sinking of the
ovarian follicles into the tissue of the ovary, instead of escaping imme-
diately, as in Fig. 18?

70. Supportive or Connective Tissues. This class of
tissues embraces the bulk of the non-active tissues in animals.
They vary much in appearance and structure, agreeing in little
except in their mesodermic origin, their passivity, and in the
prevalence of intercellular substance. The intercellular sub-
stance gives the distinctive character to the connective
tissues, the cells having a relatively unimportant place
after the production of the intercellular substance. The
general function of the supportive tissues is to bind and
sustain the more active tissues in their relations to the
body as a whole. The classification of supportive tissues

CELLULAR DIFFERENTIATION.

47

is based on differences in the intercellular substance. This
may be fluid (as in blood) or solid (as in bone) ; it may be
homogeneous (as in some forms of cartilage), or fibrous; it

FIG. 20.

FIG. 20. Cellular Connective Tissue, showing large vacuoles, v, in the protoplasm.

Questions on the figure. Would you say that these cells are of a
high or a low order of differentiation? Why? Is there any intercellular
substance? Where is tissue of this kind found? (See reference texts.)

FIG. 21.

FIG. 21. Gelatinous connective tissue, showing stellate cells (c), epithelium (*),
the gelatinous intercellular substance (j), and the intercellular fibres (f).

Questions on the figure. What seems to be the relation of the epi-
thelial layer to the tissue below it? What classes of cells are found in
the gelatinous tissue? What is their origin? What is the nature of the
intercellular substance? Are the fibres cellular or intercellular?

may be almost wholly organic, or very largely inorganic. The
principal classes are cellular connective tissues, gelatinous con-
nective tissue, fibrous connective tissue, cartilaginous tissue,
and osseous tissue.

4 ZOOLOGY.

71. Cellular or Vesicular Tissue forms an exception to the general
rule of abundant intercellular substance. It is an embryonic tissue, a
forerunner of the more permanent tissues, and is chiefly interesting from
that fact. The cells have large vacuoles or vesicles which are enveloped
by a thin layer of protoplasm (Fig. 20). It is found in the notochord
of vertebrates.

72. Gelatinous tissue has a matrix of intercellular substance envelop-
ing stellate cells, the radiating projections of which serve to connect them
Fibres are often developed in the matrix. This tissue is abundantly found
in the jelly-fish (see Fig. 21).

73. Fibrous connective tissue has in its ground substance a rich supply
of fibrils variously arranged. The cells or corpuscles are often elongated
and branched. If the intercellular fibres cross, running in various direc-
tions, a loose yielding tissue results, as in the ordinary connective tissue
about the muscles and nerves (Fig 22, A) ; if the fibres are parallel the
tissue naturally becomes more compact. There are two types of the more
compact sort differing in the quality of the fibres. The latter may be white
and inelastic, as in tendons, or yellow and elastic. Fat is frequently de-
posited as spherical drops of oil (Fig. 22, 5) in the cells of connective

tissue.-

FIG. 22.

A

Questions on the figure. In these two types of tissue which element
gives special character to the tissue, the cells or the intercellular substance ?
How would the deposition of large drops of oil in the cell affect the activity
of the cell? Why? Why are fatty deposits less hurtful amid connective
tissue than elsewhere in the body?

74. Cartilage. In cartilage the intercellular matrix is much firmer
than in those tissues already described. It may appear homogeneous as
in rib cartilage (Fig. 23, A) ; or it may contain numerous fibres which give
coherence and elasticity. The cells are usually rounded except where they
have been flattened by mutual pressure, and usually occur in pockets in

CELLULAR DIFFERENTIATION.

49

the matrix. Cartilage is bounded on its free surfaces by a fibrous mem-
brane, the pcrichondrium. This membrane assists in the growth of the
cartilage. There are no blood capillaries in cartilage.

Salts of lime may be deposited in the intercellular substance, giving it
some of the qualities of bone.

FIG. 23.

FIG. 23. Cartilage. A, Hyaline cartilage; B, fibrous cartilage. In the latter a large
portion of the intercellular substance is conspicuously fibrous. The cells occur in
pockets (p) in the matrix; /, intercellular fibres.

Questions on the figure. What are the points of similarity and of
difference in the two types of cartilage? In what manner do the multi-
cellular pockets arise? What is the nature and origin of the intercellular
substance in each case?

75. Osseous or Bony Tissue. These tissues are found only in verte-
brates, and are the most complicated of the supportive tissues. The firm
matrix which is secreted by the bone cells cpnsists of a mixture of organic
substance and inorganic matter, especially the salts of lime. The cells
with their fine filamentous branches occur more or less regularly between
thin plates or lamella of the bony material. A cross-section of one of the
long bones shows the typical condition. The periosteum is a superficial
fibrous membrane about the bone, well supplied with blood vessels. Its
inner layer of cells is capable of producing bone. Within this is a region
of firm bone, in which a series of lamellae are parallel with the surface
of the periosteum. Between the lamellae occur the spaces (lacuna) occu-
pied by the bone-cells which have been left behind as the matrix was
deposited. Deeper in the bone the lamellae and cells are in concentric
layers about the numerous blood vessels (occupying spaces known as
Haversian canals') which penetrate the bone, chiefly in a longitudinal
direction. The included bone-cells communicate with each other and with
the blood vessels by processes which occupy minute canals (canaliculi)
5

ZOOLOGY.

in the intercellular substance (Fig. 24). Within this region and imme-
diately surrounding the central cavity of the bone is often a mass of spongy
bone in which the regularity of arrangement of the cells is lost. Bone may
be formed by replacing cartilage, or wholly independent of it.

FIG. 24.

FIG. 24. Bony Tissue. A, portion of cross-section of a bone, the upper portion of
the figure representing the outer surface of the bone, just beneath the periosteum. The
open spaces, h, are Haversian canals; I, lacuna, occupied in life by bone cells. The
minute canals through the bone connecting the lacunae are canaliculi. B, a portion
of one Haversian system much magnified, h, Haversian canal, containing artery (a),
vein (v), lymphatic spaces, nutritive cells; c, canaliculi; /, lacunae; la, plate of bony
intercellular substance.

Questions on the figure. How does bone compare in appearance and
structure with the other supportive tissues? How is its intercellular sub-
stance laid down? How are the cells in the bone nourished? How do
they come to lie in the solid bone? What changes occur in this type of
tissue with age? What is the function of the Haversian canal?

Dentine and enamel, though differing in structure from bone, are to
be looked upon as belonging to the same class of tissues. They differ
chiefly in the fact that no cellular elements are included in the secretion.
They are thus harder and denser than bone.

76. We find all stages of transition between the more sim-
ple and more complex supportive tissues, and it may be seen
furthermore that there is a fundamental embryological
sequence. In the development of the organism the simpler
connective tissues give place, by transformation or substitu-
tion, to the more complex. The cellular connective tissue of
early life is replaced, for example, by cartilage, and this may
be transformed into bone in adult life.

CELLULAR DIFFERENTIATION. 5 3

77. Nutritive Fluids. The body fluids known as blood and lymph are
frequently classed among the supporting tissues, the fluid portion being
regarded as the intercellular substance and the corpuscles as the cells.

FIG. 25. Blood corpuscles (amphibian), c, colored corpuscles, flatwise and in profile;
/, colorless corpuscles (leucocytes).

FIG. 26.

FIG. 26. Blood corpuscles (human), c, colored; /, leucocytes. The red cells tend to
collect in rows with the sides in contact.

Questions on figures 25 and 26. Compare by means of the figures,
the text and reference books the colored and colorless corpuscles of
these two types of vertebrates and note the differences. In what other
respects do the colored cells differ from the white? Which are the less
highly .differentiated? Reasons for your view? Why are the colorless
corpuscles also called phagocytes?

They differ however from the ordinary tissues in the important fact that
the intercellular substance is not produced by the cells. In the vertebrates
these cells are of two kinds, the amoeboid or colorless and the colored.
Both kinds occur in the blood ; the colorless alone are found in the lymph.
The colored corpuscles are relatively numerous and are disc shaped. Re-
garded as cells they present a series of degenerative changes which results
in a loss of the distinctively protoplasmic character, by the substitution

52 ZOOLOGY.

of certain proteid substances, one of which haemoglobin is notable for
its affinity for oxygen. The degeneracy may go to the extent of the entire
loss of the nucleus, as in the mammals. The colorless cells have the
power of independent motion such as is found in the amoeba, and may ingest
solid particles of food. The body-fluids of the invertebrates contain as a
rule only colorless corpuscles, and are therefore more like the lymph of
the vertebrates. When their blood is colored it is usually from pigment
in the plasma or fluid portion of the blood. In addition to the cells the
blood carries a rich supply of proteid and other substances for use in the
tissues, and of waste products in process of removal from the body.

78. Muscular Tissue. The remaining tissues are charac-
teristically active. Muscular tissue by its contractility has the
power of producing movements of the parts to which it is
attached. This contractility of muscle may be looked upon

FIG. 27.

FIG. 27. Plain muscle fibres, n, nucleus of muscle cell; p, undifferentiated cell proto-
plasm; p', the differentiated contractile portion of the cell.

Questions on the figure. What are the two principal portions of these
cells? How do very young muscle cells compare with older ones in the
relative amount of these portions in the cell? Which is the more highly
differentiated portion? Where are such tissues found in the animal body?
Why are muscle fibres elongated?

CELLULAR DIFFERENTIATION. 53

as a specialization, and limitation in direction, of the power of
contraction which we have seen to be resident in all living
protoplasm. Muscular tissue differs somewhat in structure
and degree of differentiation in various animals, but in general
agrees in the presence of elongated fibres which are to be
considered as modified cells or parts of cells. The contractile
muscle substance is, in part at least, a plasmic product rather
than mere protoplasm; yet it differs from the plasmic inter-
cellular substance of the tissues already described in that it is
deposited within rather than among the cells. Two stages
in the differentiation of muscular substance are to be noted :
( i ) the fibres may be plain, in which case we find elongated,
contractile single cells without conspicuous external differen-
tiation (Fig. 27) ; (2) cross-striate fibres, which always show
conspicuous differentiation of parts in each fibre as seen under
the microscope. The plain fibres are characteristic of slug-
gish animals, and those parts of animals whose muscular
action is least prompt in response to the nervous stimuli (e. g.,
digestive tract in vertebrates). The cross-striated fibre usu-
ally represents several incompletely separated cells, or a multi-
nucleate condition of a much-grown and metamorphosed single
cell. In both classes the fibres are made up of numerous
minute strands or fibrilla? which in the plain muscle are homo-
geneous throughout, but in the cross-striated are made up of
alternating segments of lighter and darker optical appear-
ance (Fig. 28, B}. The undifferentiated protoplasmic rem-
nant is often very small in amount, and is collected about the
nucleus (Figs. 27, 28). It may be at the surface of the fibre
or in the centre, enveloped by the contractile matter. A thin
membrane (sarcolemma) binds the fibrillse into fibres. The
fibres are bound together by strands of connective tissue into
bundles, and of these bundles the muscle is made up.

79. Origin of Muscle Tissue. In those animals in which a true meso-
derm is wanting, the epithelial cells may develop, at their inner extremity,
contractile roots, either plain or striate (Hydra, Fig. 17, B). These cells
may wholly lose their epithelial quality and position and become entirely
muscular. In the higher animals this is very much modified by the early

54

FIG. 28. Diagram of nervous and cross-striate muscular tissue, showing the mode of
connection between nerve fibres and muscle fibres. A, nerve cell (g) connected with
muscle fibre (mf.) by nerve fibre (n./.). The muscle fibre (m./.) is composed of
numerous fibrils (/) which are made up lengthwise of alternating discs of lighter and
darker substance. These fibrils are shown more highly magnified in B and C. In
B the fibril is uncontracted ; in C it is contracted. D, nerve fibre more highly
magnified showing a, axis; m, medullary sheath; and .?, Schwann's sheath; ax., axon;
d, dendron; n, node; n.m., nerve-muscle plate.

CELLULAR DIFFERENTIATION. 55

appearance of separate mesoderm from which the whole muscular system
is derived.

80. Nervous Tissue: its Functions. The nervous tissues
are in close relation on the one hand to the sensory epithelium
and on the other to the muscular tissue. Through the former
they receive the stimuli from the outside world; by means of
their connection with the latter they are enabled to effect
responses. The reception of stimuli, the transmission of the
results of stimulation, and the initiation of appropriate re-
sponses constitute the fundamental work of nervous tissue
(Fig. 28, A, D}. In some of the lower Metazoa the same cell
may do all these tasks.

81. Structure. The principal elements of nervous tissue
are the nerve-cells (ganglion-cells} and nerve- fibres. The
cells, which are the centres of nervous activity, are usually
large with conspicuous nuclei. The fibres are, in their essen-
tial parts merely outgrowths of the ganglion-cells. These out-
growths are of two sorts : the dendron, which is a much, and
irregularly, branched structure ; and the axon, or nervous fibre
proper. Each cell may have one or more processes arising
from it. These fibres may pass just as they arise from the
cells, without special structural modification, to their connec-
tions. Such are called non-medullated or gray fibres. There
are usually however one or more protective sheaths formed
about this essential axis: (i) the medullary sheath, consisting
of a framework filled with a fatty material, surrounded by
(2) Schwann's sheath, a homogeneous sheath with occasional
nuclei along its course (Fig. 28, D}. Fibres possessing the
medullary sheath are called medullated or white fibres.

Questions on Fig. 28. What are the principal points of contrast
between the plain and the cross-striate muscular fibres? Enumerate the
principal regions of the nerve cell figured. How does it differ from a
typical cell in form? What are the principal parts of the nerve-fibre (D) ?
What are the supposed functions of these various portions? Why is it
necessary for nerve cells to be in connection with other kinds of cells?
What are the differences between the contracted and uncontracted muscle
fibre (B and C) ? What is meant by a neurone?

50 ZOOLOGY.

A nerve cell together with its processes is called a neurone.
The whole nervous system may be considered as made up of
such units, which connect with each other by the delicate
terminal branches of the outgrowths. See Fig. 34.

82. Origin of Nervous Tissue. Nervous tissue always arises from
the ectoderm of the embryo, so far as we know. In some of the lower
forms of animals, as the coelenterata, the nervous cells may be derived
individually from the epithelium. In such instances they have a close
connection with those muscle elements which are also of epithelial
origin (see 79). In the higher forms the origin of the nervous mat-
ter from the ectoderm is somewhat less direct but essentially similar.
The connection of the nervous centres with the muscles and glands,
etc., in the higher animals is a secondary condition and is the result
of the growth of the nerve fibres toward such organs. What directs
their growth to the right place when the fibres begin to grow, we do
not know.

83. Summary.

1. The individual becomes complex by the increase of the
number of cells, and by their differentiation.

2. A tissue consists of a group of similar cells with their
products, which are adapted to the performance of special
work or function.

3. Tissues differ morphologically in respect to the form,
arrangement, and structure of the cells, and in the amount,
arrangement and consistency of the intercellular substance.

4. Physiological differentiation accompanies the morpho-
logical, the division of labor becoming very complete in the
higher forms. The physiological value of a tissue may de-
pend either upon the cells or the intercellular substance.

5. Tissues may be classified as follows :
A. The vegetative or passive tissues.

I. Epithelial :

function : protection, absorption, secretion,

sensation, reproduction, etc.
kind : pavement, columnar, ciliate, glandular,

sensory, muscular, reproductive, etc.

II. Supportive or connective:

function : binding, support, protection.

CELLULAR DIFFERENTIATION. 57

character : abundant intercellular substance,
form: vesicular, gelatinous, fibrous, cartilagi-
nous, osseous, etc. (blood and lymph).
B. The active tissues.

III. Muscular:

function : irritability, especially to nervous
stimuli ; contraction in a definite direction.

form: plain and cross-striate (depending on
the differentiation of the contractile sub-
stance).

IV. Nervous :

function : reception of general stimuli,

transmission of impulses, interpretation,

arid the initiation of appropriate responses.

form: central cells (ganglion) with fibrous

branches (axon, dendron).

6. The epithelial tissues arise 'from ectoderm, entoderm and
mesoderm ; the connective tissue, from mesoderm ; the muscu-
lar, chiefly from mesoderm; and the nervous tissue, from
ectoderm.

84. Exercises for the Laboratory (these may be made as
extensive as time and facilities will allow).

i. Temporary demonstrations of the simpler tissues should
be made by the teacher or pupil, by teasing out with needles
small portions of the appropriate material in a drop of water
on the slide.

(a) Blood. Compare that of earth-worm or insect, frog,
man. Place a drop of fresh blood on the slide and cover.
Examine at once. The teacher should have a permanent
preparation of the blood of the frog, stained to show nucleus
of corpuscles.

(b) Epithelium. Mesentery of cat; film shed from skin
of frog kept a few days in captivity; cells scraped from the
oesophagus of a recently killed frog.

(c) Connective Tissue. Found surrounding muscle, i. e.,
lean meat. Compare tendon.

58 ZOOLOGY.

(d) Muscle. From wall of stomach, from heart, from
skeletal muscles.

(e) Nerve Fibres. Small portion of nerve of frog or cat.
2. The teacher should secure permanent mounts of sections

of cartilage, bone, and tooth showing dentine and enamel.
Properly stained preparations of glandular tissue, of nerve
cells and their branches, and of reproductive epithelium (see
appendix), will greatly assist the pupils in securing an accurate
idea of these tissues and their work.

CHAPTER VI.

THE GENERAL ANIMAL FUNCTIONS, AND THEIR
APPROPRIATE ORGANS.

85. Protoplasmic Functions. It has already been stated
that in protoplasm reside the fundamental powers belonging
to living things. Through its agency all the metabolic or
assimilative processes are performed. Through the activity
of its ferments foods undergo changes that prepare them to
be used in the manufacture of protoplasm and other complex
cell-substances. In protoplasm occur the oxidation and other
chemical changes which result in the manifestation of energy,
as heat, motion, light, etc., accompanied by the formation of
waste products which are to be eliminated. The power of
receiving and responding to surrounding influences, which we
have called irritability and contractility, is likewise a power
of protoplasm. Out of these arises the possibility of organ-
isms becoming adapted to their surroundings. It is not yet
possible certainly to localize all these functions within the
individual cell, although it seems probable that in some degree
even such protoplasm as is found in the Amoeba has localized
functions. In many single-celled animals there is a consider-
able localization.

86. Division of Labor. As the protoplasmic units, i. e.,
the cells, increase in number by cell division and form large
masses, they are no longer subjected to the same influences,
and are not equally favorably situated for the performance of
all the original functions. The protoplasm of all cells retains
the power of using food and of building up its own substance,
but we find certain activities largely given over to special
groups of cells ; e. g., secretion is specially noticeable in some,
contractility in some, and irritability in others. This division
of labor, is accompanied by a corresponding differentiation of

59

60 ZOOLOGY.

structure which constitutes an adaptation to the special work
to be done, and is of great advantage. We have described
these structure-groups as tissues (see Chapter V.).

87. Organs. The tissues which have been described are
never independent, but are associated with each other in the per-
formance of a common function, to form an organ. In each
organ there is usually a principal tissue which determines its
function (as muscular tissue in muscle, or the glandular tissue
in glands), and one or more accessory tissues for support or
control (as connective or nervous tissue in the organs men-
tioned). To accomplish some of the activities, in the higher
animals especially, several organs of a similar kind must work
together. These are sometimes spoken of collectively as sys-
tems of organs, e. g., digestive system, circulatory system,
and the like.

88. Classification of the Systems of Organs and Func-
tions. The work that needs to be done by an organism may
be considered under the following heads: (i) metabolism
including digestion, circulation, assimilation, respiration, and
excretion; (2) protection and physical support; (3) growth;
(4) reproduction; (5) movement; (6) sensation. Eight sys-
tems of organs can be distinguished by which this work is
done. They are (i) the digestive; (2) circulatory; (3)
respiratory; (4) excretory; (5) skeletal and integumentary;
(6) reproductive; (7) muscular, and (8) nervous.

89. Metabolism (Nutrition). Metabolism embraces two
sets of processes, (i) constructive or anabolic, known as
assimilation, and (2) destructive or katabolic. By construc-
tive we mean all the. processes in the organism which result in
the storing of food and energy, in growth, repair, and repro-
duction. We class as destructive all those processes by which
the complex cell substances are broken down and energy set
free, leading to change of temperature, to nervous or muscular
action, to secretion and excretion. In the higher animals the
nutritive process is a very complicated one and demands the

THE GENERAL ANIMAL FUNCTIONS. 6 1

cooperation of numerous organs. It embraces the ingestion
or taking- in of food; the digestion of food; its absorption
from the digestive tract into the body fluids blood and lymph ;
and its transportation in these systems, which is made neces-
sary by the fact that digestion is confined to a special region.
It likewise includes the further absorption of these materials
from the blood and lymph by the cells for whose benefit all
the preceding work has been done; the assimilative process
within the cell whereby the food material is made into proto-
plasm or other complex cell-products ; the reception and trans-
mission of oxygen, by the combining power of which (oxida-
tion; see 25) these complex substances are broken down
into simpler ones useful, useless, or hurtful to the animal
economy. Finally, the elimination of the products of this
oxidation or burning is a necessary part of the nutritive
process. If the material eliminated from the cell is of further
use the process is known as secretion, if not, excretion. It is
undesirable to attempt to make a sharp distinction between
excretion and secretion.

90. The Digestive System. The simplest condition of
the digestive tract is found in the gastrula (archenteron, Fig.
n, 4) or in Hydra (Fig. 79). Here there is only one cavity
in the body and the food is taken up immediately by the cells
needing it. A simple modification of this condition is seen
in Fig. 29. A still more complicated condition is shown in
Fig. 93. In this form which we may take as the type, the
digestive tract is a tube, running through the body, lined with
its own epithelium and is separated from the body wall by
the coclom or body cavity. The tube itself may have any
degree of complexity, but consists essentially of (i) an an-
terior portion (stomodceum} lined with ectoderm, (2) a pos-
terior portion (proctodazum} also lined with ectoderm, and
(3)3. middle portion (mesenteron) lined with entoderm. The
stomodceum or mouth region is usually supplied with devices
for the capture and ingestion of the food. The mesenteron
is the true digestive region. It is supplied with cells which

62 ZOOLOGY.

secrete materials which act upon foods in such a way as to
render them capable of being absorbed through the entodermic
cells into the body cavity, or that special portion of it known
as the circulatory system. Pouches and outgrowths from the

FIG. 29.

FIG. 29. Stenostoma (after Hertwig). In this Turbellarian the digestive tract (.d.t.)
is a blind sac. st., boundary of stomodseum and mesenteron; c, cilia; g, ganglion
(brain); g', ganglion of a new individual which is being formed by fission; o, mouth; o',
mouth of new individual in process of formation; w, excretory system.

Questions on the figure. How much of this digestive tract is lined
with ectoderm? Which portion with entoderm? Is there a proctodaeum?
What are the evidences that the worm is in process of division? Compare
this digestive tract with those in Figs. 79, 85, 93, 99.

wall of the mesenteron are of common occurrence. These
serve to increase the glandular or secreting surface, the ab-
sorbent surface, and also to retain the food longer in contact
therewith by retarding its passage through the canal. The
removal of the digested food from the canal may be effected
by absorption or by the active engulfing of food by the ento-
dermal cells, much as is done by the amoeba.

91. The Respiratory System and Function. In addition
to its other food requirements, all protoplasm, in proportion
to its activity, must have free oxygen. This is obtainable from
the air or from the oxygen dissolved in water. Oxygen, being
a gas, must enter the system in a somewhat different way from
that by which fluids and solids are ingested. It is best obtained

THE GENERAL ANIMAL FUNCTIONS. 63

by absorption through moist, thin-walled membranes. Such
surfaces, in connection with which blood vessels are usually
found, constitute the respiratory organs. Any exposed sur-
face meeting these requirements may serve as such. The
general surface of all animals is respiratory in some degree.
In the more complex animals, however, special additional
organs must be provided. This may be effected by thin out-

FIG. 30. FIG. 31.

b. c. *C3fc05CBN ex.

FIG. 30. Diagram illustrating gills or branchiae, b.c., cavity in which the body fluids
circulate; br., branchial filaments which are merely much thinned out-pocketings of the
body wall (a/) ; ex, the external medium water in which the oxygen is dissolved.

Question on the figure. What are the essential features of gills as
suggested by this figure? Why are they better suited to water than to air?

FIG. 31. Diagram illustrating lungs or tracheae, b.c., the cavity in which the
body fluids circulate; /, the walls of the lung, which are much thinned inpocketings
of the body wall (a;) ; ex., the external medium usually the atmosphere in which
the oxygen is found.

Questions on the figure. What are the essential features of lungs as
suggested by the figure? Why are such organs better suited to aerial than,
to aquatic life? In what respects are gills and lungs better than the mere
body-wall for the exchange of the gases?

growths of the body surfaces, which are especially adapted
to water forms and are called gills or branchice (Fig. 30) ; or a
similar increase may be attained by pits or ingrowths of the
body surface, suited to get oxygen from the air. Such' are
called lungs (or trachea} (Fig. 31). Carbon dioxid, a gas-
eous waste product resulting from the union of oxygen with
carbon which takes place in the tissues, is economically elimi-

04 ZOOLOGY.

nated by the same organ which admits the oxygen, inasmuch
as the entrance of one gas is not retarded by the outward pass-
age of the other. This double process constitutes respiration,
although the latter half is also appropriately described as ex-
cretory. The surface devoted to the exchange of the gases
and the special devices necessary to renew the air or water
make up the respiratory system. The respiratory organs are
frequently associated with the anterior or posterior end of the
digestive tract. As in the case of other foods, the blood is the
vehicle by which oxygen is distributed from the gills or lungs
to the parts of the body needing it.

92. The Circulatory System and Function. In such con-
ditions as are shown in Fig. 79, there is no circulatory system.
The digested food is merely distributed from cell to cell. In
animals in which the digestive apparatus is well developed,
some device becomes necessary for the distribution of the
food. The body cavity with its contained fluids may do this
work as in Fig. 29. Usually however when the mesodermal
layers become well developed, there arises therefrom a series
of branching tubes containing special fluids, blood or lymph.
These tubes by their ramifications connect the digestive sur-
faces with the various parts of the body. Some branches like-
wise extend to those special surfaces where the oxygen of the
external medium may be had. Naturally then the complexity
and the special structure of the circulatory system depend
largely upon the position and degree of development of the
digestive and respiratory organs. In order to secure the neces-
sary motion of the fluids contained in the tubes, the walls of
the latter are supplied with muscular fibres, and contract more
or less rhythmically. If the motion is to have a definitely con-
tinuous direction, as is ordinarily the case, valves are usually
so placed that motion in the opposite direction will be im-
possible. The (one or more) contractile regions are called
hearts; vessels conducting blood from the heart are arteries,
those carrying blood toward the heart, veins. In the region
where the vessels are smallest and have very thin walls, the

THE GENERAL ANIMAL FUNCTIONS. 65

exchanges between the blood and the other tissues occur. This
is the region of capillaries. The blood system has capillaries
in the walls of the digestive tract, in the respiratory organs,
and in and about all the tissues receiving a direct blood supply.
The capillary region is that for which the rest exists ; it is the

FIG. 32.

FIG. 32. A scheme to represent the circulation of the blood, in its essential features.
The arrows indicate the course of the blood, a, arteries; aur., auricle or receiving por-
tion of the heart; d, digestive tract; c. d., capillaries of the digestive tract; c.r., capil-
laries of the respiratory organs; c.s., capillaries of the system; va., valves; re, veins;
rt., ventricle.

Questions on the figure. What portions of the apparatus are neces-
sary to secure circulation? Which secure the real objects for which the
circulation exists? Why are valves essential? What common work occurs
in the three classes of capillaries figured above? What special type of
work is characteristic of each of the three?

physiologically important part of the system. Fig. 32 illus-
trates the arrangement of parts found in a common type of
circulatory apparatus.

93. Demonstration. Circulation of blood in tail of tadpole; in the
web of the foot of a frog; or in the fin of small fish. Distinguish veins
and arteries. Notice behavior of corpuscles in passing through small capil-
laries. Compare rate of flow in vessels of different size.

94. The Excretory System and Function. Beside the
carbon dioxid eliminated from the blood in the lungs or gills,
other waste products of oxidation are to be removed from the
tissues where they are produced. Important among these are

66

ZOOLOGY.

the nitrogenous wastes, urea and uric odd. In organisms in
which there is no regular blood system, these waste products
may be carried directly from the tissues to the surface by a
system of tubes beginning as capillaries. In the majority of
animals the canals (nephridia) pass from the body cavity to
the exterior. These are seen in a simple condition (Fig. 33)

FIG. 33.

FIG. 33. Diagram of a nephridium (simple kidney tubule) of a Segmented Worm.
b., V., blood vessels; c, coelome; d, duct of the nephridium; e, external opening; cf,
ciliated funnel opening into coelome; gl., glandular or secreting portion; s, septum;
W, body wall composed of longitudinal muscle fibres, circular fibres, and epithelial
layer; w, wall of gut.

Questions on the figure. Judging from its relation to the coelome, to
the blood vessels, and to the outside world what would seem a reasonable
function for the nephridium?

in the segmented worms. For a more complicated condition
see unsegmented worms (Figs. 86-88). The kidneys of
higher forms are considered to be derived from these. In the
higher animals the kidneys have a special blood supply and
the waste products are extracted from the blood while in the
kidney.

95. The Skeletal System and its Functions. The cells of
the body frequently excrete from themselves materials which,
while no longer of use to the protoplasm, are not entirely re-
moved from the body by the blood and serve important passive
functions. These excretions or secretions may surround and
protect and give rigidity to the cell itself (e. g., cell-walls;
shells in the single-celled animals), may bind the cells together

THE GENERAL ANIMAL FUNCTIONS. 67

into a resistant tissue (intercellular substance in bone, etc.),
or may be secreted at the surface of the organism as a whole
(cuticula in insects and shell in mollusks). The hard parts
serve primarily for the support and protection of the softer
tissues. Incidentally they come to serve a very important use
as points of attachment for muscles. The skeleton may be
external (the integumentary skeleton, or exoskeleton) as in
crayfish, or internal, as the endoskcleton of vertebrates. In
many instances both kinds of skeletal structures may occur
simultaneously, yet it is usually true that if the exoskeleton
is well developed the endoskeleton will be poorly represented.
Each has important advantages and limitations. To allow
motion as the result of muscular action the skeleton, if rigid,
evidently must be in segments and jointed.

96. Growth. There are no special organs of growth, yet
growth is one o-f the most immediate and important manifesta-
tions of the nutritive process. Growth is to be defined as in-
crease in volume or mass and may result from either or all of
three processes: viz. (i) absorption of water, (2) formation
of protoplasm and the multiplication of cells, and (3) forma-
tion of non-protoplasmic cell-products, either within or among
the cells.

The rate and character of growth are modified by such
external conditions as temperature, light, quantity and quality
of the food supply, etc. Growth does not continue indefinitely.
Its continuance is determined by the relation of the anabolic
to the katabolic processes in the body. The time comes in the
life of every complex organism when the income no longer
equals the outgo, and growth must cease. Later still the wear
is not made good by the income, and death results.

97. Reproduction and the Reproductive Organs. Since
individual organisms are limited both with regard to growth
and length of life, it is apparent that a given class of forms
cannot continue, unless some method of originating new in-
dividuals be found. This production of new individuals by

68

ZOOLOGY.

the instrumentality of the old is reproduction. In many of
the lower animals this is merely a growth process, " growth
beyond the limits of the individual." In the single-celled ani-
mals reproduction means the formation of the protoplasm into
two or more masses, by dividing into two equal parts (divis-
ion), by breaking into a large number of sub-equal portions
(fragmentation), or by budding (Chapter III, 39). In bud-
ding there is the formation of a local outgrowth which ulti-
mately attains the size and character of the parent. In division
the resulting individuals cannot be distinguished as parent and
offspring. Such reproduction, involving only one parent, is
asexual. It usually occurs when the adult size of the animal
is attained. It is not confined to the Protozoa or single-celled
animals, but may occur in several Invertebrate groups in which
(Hydra, Fig. 79) there is not a high degree of specialization.
The budded individual or offspring may in such cases consist
of one cell or of many. In addition to the stimulus afforded
by the attainment of normal size, external conditions such as
diminished food supply, temperature changes, etc., influence
the process of non-sexual reproduction.

98. Sexual Reproduction. It seems for some reason that
even in the one-celled animals the method of reproduction by
division cannot be continued indefinitely without ill effects to
the organism. In many Protozoa there is at certain times a
union of two individuals, either temporarily or permanently,
accompanied by exchange of nuclear material or a fusion of
the whole protoplasm. After a period of rest division begins
again with renewed activity. Something similar is seen in
the more complex animals the Metazoa. After a period of
cell divisions, by which the individual body is built up, the
majority of cells, as muscle or nerve cells, appear to lose their
power of dividing, and even the less differentiated cells which
we have described as the ova and sperm are incapable of con-
tinuing the division necessary to produce a new individual until
they have been stimulated by union with each other (or by
some artificial means). Such unions of cells, to form by later

THE GENERAL ANIMAL FUNCTIONS. 69

divisions a new individual, are called conjugation or fertilisa-
tion, and the new individual which results is said to arise by
sexual reproduction. The uniting cells may be similar (as in
Pandorina), in which case the union is isogamous. More
usually the cells are different and the union is heterogarnous.
In the latter case the cells are called ovum and sperm (Chapter
IV) and are usually formed by different individuals, though
very often the same individual may produce both classes of cells
(hermaphroditism) from different regions of the germinative
epithelium, or in the same organ at different times. The
special organs in which the ova are produced are called ovaries.
The sperm cells are formed in testes. The individuals (that
is, the male and female) producing the different classes of
cells are often very different in other respects. This is known
as sexual dimorphism (Chapter VII, 145).

99. Practical Exercises. Compare the males and females of the vari-
ous animal types with which you are familiar. In what groups of animals
does non-sexual reproduction occur? Give the gist of Geddes and Thomp-
son's theory as to the origin of sexuality. Compare any other theories
available to you. What are the conceivable advantages and disadvantages
of the asexual method? of the sexual? of hermaphroditism? of sexual
dimorphism?

100. Movement and the Muscular System. The desir-
ability of motion in animals arises from the necessity of seek-
ing food and of escaping unfavorable influences. These con-
ditions constitute the most imperious stimuli to which the or-
ganism is subject. We have already seen ( 19, 23) that the
fundamental irritability and contractility of protoplasm make
this response possible in the simplest conditions. In somewhat
higher forms, specially developed protoplasmic fibrils appear,
such as cilia, or the fibrils in the stalk of Vorticella, in which
the power of contracting is strikingly manifest (see Figs. 66
and 68). While this is found in Protozoa, it is much mo^e
clearly show r n in the muscular tissue (Fig. 28) of still higher
animals. Locomotion varies in efficiency in different animals
not merely on account of varying muscular structure but in
accordance with the arrangement of the hard parts to which

7 ZOOLOGY.

the muscle fibres are attached, and the nature of the medium
which must be penetrated. Many aquatic forms, though free-
swimming in their early stages, may become attached and give
up the power of locomotion in the adult condition. Such
attached forms ordinarily secrete an external shell or covering
into which they can withdraw for protection (e. g., barnacles,
many polyps). They must depend upon currents in the water
to supply them with food. They are frequently able to produce
the currents by the motion of parts of the body. The majority
of active movers have hard parts which serve as levers to
which the muscles are attached. The parts of the skeleton,
which may be either external to the muscles or surrounded by
them, articulate with one another by a hinge or movable joint,
as illustrated by vertebrates or insects. In some forms with-
out a conspicuous skeleton, as the earthworm, there is a
dermo-nmscular wall surrounding a fluid-filled cavity. By the
alternate use of the longitudinal and circular fibres, changing
the relative position of the parts of the body, locomotion is
effected. The special appendages, particularly the paired
appendages, are important motor organs in nearly all actively
moving animals.

101. Sensation and Sensory Structures. In a simple bit
of protoplasm it is manifest that the differences between the
living matter and the outside world are greater than the struc-
tural differences between the parts of the protoplasm itself.
Thus we would expect the stimuli arising from the environ-
ment to be among the most important experienced by the
organism, and that the superficial protoplasm by virtue of its
irritability (see also 19) would most promptly feel and
respond to such stimuli. The changes thus instituted will be
felt sooner or later to the remotest parts of the cell mass.
This transfer of the effects of a stimulus through a longer or
shorter distance introduces us to a second nervous function,
internal irritability of protoplasm or conductivity.

1 02. As an organism increases in the number and variety
of its cells, the specialized structures need to be more com-

THE GENERAL ANIMAL FUNCTIONS. 71

pletely bound to one another. It becomes necessary not only
that they receive impulses from such parts as are favorably
situated for the reception of stimuli, but that a degree of
coordination of the interrelated parts be secured, in order that
just such response shall be made as will best meet the needs
of the organism. This power of coordinated response to ex-
ternal stimuli makes it possible for an organism to become
suited to its environment.

103. In the higher forms, the work above described de-
mands five classes of structures (see Fig. 34); (i) end or
sense organs, which are specially sensitive to stimuli of dif-
ferent orders, as mechanical (touch), chemical, ethereal
(light), etc.; (2) conductive tracts (afferent nerves), which

FIG. 34-

FIG. 34. Scheme showing the essential relations of the parts of a nervous system: I,
the sensory end organ (epithelial); 2, afferent nerve tract; 3, central nervous cells
(ganglia); 4, efferent nerves, leading to 5, muscle, gland, etc. g, ganglion cells; gl.,
gland; m, muscle fibre; n.f., nerve fibre; s.e., sensory epithelium.

Questions on the figure. What seems to be the function of the
various parts or elements in this scheme? Your reasons for your view?

connect (i) with (3) central nervous structures (ganglion-
cells) where the impulse is received and suitable responsive
impulses are originated; (4) conductive tracts (efferent
nerves), which make the work of the central organs of value
by carrying an impulse which produces corresponding activi-

ZOOLOGY.

ties in (5) some form of related cells: muscular, glandular,
or nervous. It is readily apparent how increase of volume and
differentiation of the other parts will make necessary a more
complicated nervous system. The special arrangement of these
parts of a complete system differs very much in various animal
groups, yet it may be said that there is a progressive accumula-
tion of the central nervous matter at the anterior end of the
body as we ascend the scale of animal life. When this con-
centration is well advanced the mass of nervous matter is
called the central nervous system which always includes the
brain. The nerves passing to and from the central part and
their endings, taken collectively, are described as the peripheral
nervous system.

104. Arrangement of the Central Nervous System. The
ganglion cells composing the nervous system may be so scat-
tered through the superficial layers as scarcely to deserve the
name " central " (Hydra). The nerve cells may be arranged

FIG. 35.

FIG. 35. Diagram showing arrangement of the nervous matter in Starfish, c, gangli-

onated ring about the mouth; o, mouth; r.n., radial nerve in each arm.

FIG. 36. The nervous system of the Clam, from the dorsal aspect, a, anterior;

o, mouth; e.g., cerebral ganglia (brain); p.g, pedal ganglia; v.g., visceral ganglia.

Questions on Fig. 35. Describe in your own terms the way in
which the principal nerve elements are arranged in the starfish. Compare
it with those which follow. In what respects similar? In what unlike
them?

THE GENERAL ANIMAL FUNCTIONS.

73

in a ring about the mouth, or gullet, with or without addi-
tional bands of nervous tissue containing cells passing radially
from it (as in echinoderms and some ccelenterates ; see Fig.
35). In the higher invertebrates this process of concentration
continues and the ganglionic cells are collected into two or
more ganglia connected by nerve fibres (commissures). Usu-

FIG. 37-

FIG. 37. Arrangement of the nervous material in the anterior end of an Oligochetc
Worm, seen in profile. That part of the body wall nearest the observer is supposed to
be removed, a, anterior; b.w., body wall; g, dorsal ganglia (brain); g', ventral chain
of ganglia; n, nerve ring around the pharynx; o, mouth; p, pharynx.

FIG. 38. The central nervous system in a Leech. Lettering as in Fig. 37.

Questions on figures 36, 37, and 38. How is the nervous matter re-
lated to the digestive tract and to the animal as a whole in all of these
figures? Compare with figures in other texts. Is there any apparent
correlation between the form, symmetry or segmentation of the animal
and the arrangement of the nervous material? Can you state your con-
clusion as a law?

ally a pair of ganglia occurs in the region of the mouth, and
dorsal thereto (e. g., clam, Fig. 36). In segmented forms,
as the earthworm and crayfish, there is also a series of gang-
lia connected by fibres, ventral to the digestive tract. This
chain is in turn connected with the dorsal ganglia by a loop
of fibres passing round the oesophagus (Figs. 37 and 38).

74 ZOOLOGY.

In vertebrates the central nervous system consists primarily
of a tube with nervous walls the spinal cord which may be
specially enlarged and thickened at the anterior end to produce
the brain (Fig. 170). From the various parts of this cord the
nerves take their origin.

105. The Peripheral Nervous System: Sense Organs.
We know by experimentation that in the lowest animals even,
or for that matter, in protoplasm, certain external conditions
produce definite responses or changes. We also know that
these external happenings and their responses, in our own
case, are accompanied by certain sensations, as touch or taste.
By inference, both from the nature of the response and from
the structure of organs, we reach the conclusion that the lower
vertebrates and higher invertebrates experience sensations in
some degree similar to ouY own. The classes of possible
stimuli have already been mentioned ( 20). Those producing
in us definite sensations are : simple contact stimuli, producing
the sensation of touch and pressure ; vibratory contacts, giving
rise to hearing and temperature sensations; chemical actions,
making possible sensations of taste and smell ; ethereal vibra-
tions, producing the sensation of light. In the lowest forms
of animals there are no specialized organs for the reception of
particular stimuli, and in such cases it is reasonable to infer
that the distinctness of the sensation cannot be very great. In
almost all animals, however, certain areas are specially suited
to be stimulated by special stimuli.

106. Touch. There are two principal ways by which contact stimuli
are received among animals. Fibres of the central nervous system may
pass to the skin and end. among its outer layers as free nerve terminations,
or these fibres may become intimately united with one or more of the
cells of the epithelium. The most common of the tactile organs in verte-
brates are of the first class. Where the stimulus reaches the nerve through
a nervous epithelium, the epithelial cells often have special developments
such as hairs, bristles, and the like, whereby the possibility of contact with
external objects is increased. The appreciation of changes in temperature
is also associated with the general skin surface.

107. Chemical Sense (including taste and smell). It is impossible
for us to distinguish between taste and smell in the lower animals. Indeed

THE GENERAL ANIMAL FUNCTIONS. 75

it is with difficulty that we separate the sensations obtained from the two
sources even in our own case, in some instances. Almost all animals seem
to have some power of appreciating the chemical condition of the medium
in which they live. In aquatic animals the chemical sense organs may be
distributed over the surface of the body. In the higher animals they col-
lect more and more at the anterior or mouth-end of the animal, with
manifest advantage to the animal. In the higher land forms, especially
the vertebrates, the organs of the chemical sense come to lie in or about
the mouth and nose, the beginnings of the digestive and respiratory tracts
respectively. These senses are specially related to the testing of food and
the medium in which the animal lives. The senses thus far enumerated
seem among the earliest developed in the animal kingdom.

108. Hearing. It is by no means certain that the lower animals pos-
sess the ability to appreciate the vibrations in matter (air, water, etc.),
which arouse in us the sensation of sound. There are in several groups
of such animals organs, the structure of which would suggest that they
might receive vibrations of the medium in which they live. In their sim-
plest condition they consist of a sac (otocyst) derived from the ectoderm
and lined by an epithelium containing sensory cells. From these cells

FIG. 39.

s.c.

FIG. 39. Otocyst in a Mollusk. n, nerve; o, otolith; s.c., sensory cells in wall of
otocyst. (After Claus.)

Questions on the figure. What immediately stimulates the sensory
epithelium in this case? What kinds of general agencies might be sup-
posed to produce the necessary motion for this purpose? What is the
present view of the function of otocysts?

sensory hairs extend into the cavity (Fig. 39). The cavity contains a
fluid which may support one or more solid particles (otoliths). With the
vibration of the medium the whole would be put into vibration, but the
inertia of the contained fluid and otoliths would cause the latter to strike
against the hairs and thus serve as stimuli to the sensory cells. Late
researches tend to prove that these structures are organs for preserving
equilibrium rather than of hearing. In higher forms the ear becomes im-
mensely more complex, but the general conditions both of origin and

7 6

ZOOLOGY.

structure appear to be much as described for the otocysts. In some of the
lower animals, as insects, there are also external vibratile hairs which are
believed to be auditory (Fig. 40).

FIG. 40.

FIG. 40. Antenna of Male Mosquito (Culex pipiens). By J. W. Folsom.

Questions on the figure. Compare with the antennae of a female (see
Fig. 63). What are the differences between the head of the male and
female mosquitoes? What is believed to be the function of these plumose
antennae? What are the evidences for this view?

109. Sight. There are three distinct facts to be noted with respect to
visual sensation in the higher forms of animals : the perception of light,
the perception of color (f. e., light of different wave-frequency) and the
formation of images of external objects. It has already been seen (20)
that protoplasm is sensitive and responsive to light without any special
organs. The simplest visual organs found in multicellular animals con-
sist merely of epithelial cells containing pigment in which changes are
wrought apparently by the action of light (Fig. 41, a). These changes
affect the nerve fibres associated with the pigment cells and thus the
central nervous organ. Such eyes are capable only of giving knowledge
of the intensity or, if properly constructed, of intensity and direction of
the light and do not form an image of external objects. There are
several types of image-forming eyes in the animal kingdom. The most
familiar of these is the "camera eye" of vertebrates, so called because

THE GENERAL ANIMAL FUNCTIONS.
FIG. 41.

77

FIG. 41. Diagrams showing some of the stages in the increasing complexity of the
simple eye in Invertebrates. A, simple pigment spot in epithelium having nerve-endings
associated with pigment cells (as in some medusae) ; B, pigment cells in a pit-like de-
pression (as in Patella) ; C, with pin-hole opening and vitreous humor in cavity (as
in Trochus) ; D, completely closed pit, with lens and cornea (as in Triton and many
other Mollusks) ; E, pigment area elevated instead of depressed, lens of thickened
cuticula (as in the Medusa, Lizsia) ', F, retinal cells more highly magnified, ep.,
epidermis; f, nerve fibre; /, lens; op, optic nerve; p, pigment cells; r, retina; v.h.,
vitreous humor.

Questions on the figures. What changes take place in the sensory
epithelium in this series of figures? What is gained by such a depression
as occurs in 5? What purpose is served by the pinhole and the vitreous
humor of C? Describe the change from C to D. What is gained? What
may be the function of the pigment? Compare texts. In what respects
does E differ from the other types? What two types of cells are figured
as belonging to each retina? What constitutes the retina?

it illustrates the principles made use of in the construction of the photo-
graphic camera. In this there is a lens or body which refracts the rays
of light in such a way that all the rays passing from a point in the object
are brought to a focus at a point on the retina. Another type of image-
forming eye is the compound eye of insects and Crustacea (Fig. 42).
These are made up of a large number of eye elements each structurally
compjete in itself whose separately formed images must nevertheless be
joined in order to form a picture.

The degree to which the color-sense is developed among lower animals
is very uncertain. The simplest animals may respond differently to light
of different colors, but this is a very different thing from saying that they
possess the color-sense.

7 8

ZOOLOGY.

To summarize, the essential part of the eye is the sensitive layer
known as the retina. The other parts of the complex eye-structure serve
the purposes of shutting out the light except from certain directions; of
focusing the light admitted in such a way as to increase its intensity and
form an image on the retina; of adjusting this apparatus to objects at

FIG. 42.

_H n

FIG. .42 Diagram illustrating the compound eye of arthropods. A, the whole eye
shown in section; B, one of the eye-elements (ommatidium) more highly magnified, c,
cuticular facets; ep, epidermis; /, group of cells forming lens-like body; n, optic nerve
fibres; o, optic ganglia; p, pigment cells.

Questions on the figure. In what way is the independence of each
ommatidium secured? In other words what is to prevent the light which
comes in obliquely from passing from one ommatidium to another? In
what conceivable way is a general image obtained from these various
partial views? What groups of animals possess eyes of this sort? Compare
the diagram B with the figure of the complete ommatidium of the lobster
(Fig. 125).

different distan'ces ; of nourishing and supporting the more important
portions "of the apparatus ; and of moving the eye so as to take into view
different portions of the surroundings. Some of the various grades of
compexity of eye-structure in the Invertebrate series beginning with a
pigment spot and ending with a complete lens-eye, are shown in Fig. 41.

no. Analogy and Homology. In comparing various
animals we find that they may do the same work with organs
that arise in very different ways, which however, because they
are adapted to perform similar tasks, look somewhat alike.

THE GENERAL ANIMAL FUNCTIONS.

79

Such structures are said to be analogous (as the wing of a
bird and the wing of a butterfly). In other cases organs
that originate in the same way may have been used so differ-
ently as to have a very different appearance, as the various

FIG. 43.

FIG. 43. Diagrams illustrating two stages in the development of the vertebrate eye.
A, showing the relation of the ectoderm, the brain vesicle, and the optic vesicle. The
right side of the figure shows a later stage than the left. B, later stage, showing
the lens, eye-ball and retina in position, b.v., brain vesicle formed by the invagina-
tion of the ectoderm (ect.) ; I, lens; mes., mesodermal tissue; o.n., optic nerve; o.s.,
optic stalk; o.r., optic vesicle, a portion of the brain vesicle; r, retinal layer; v.h.,
interior of eye-ball which comes to contain the vitreous humor.

Questions on the figures. Which portions of the eye are derived
directly from the ectoderm? Which indirectly, i. e., from the brain?
Which portions seem of mesodermal origin? By following the invagination
by which the retina is formed do you find any suggestion of an explanation
of the fact that the sensitive portion of the retina (rods and cones, Fig.
173) is directed away from the light? Refer to some work on the embry-
ology of the vertebrates for more complete series of figures.

" legs " of the crayfish, or the wing of a bird and the arm
of man. These, notwithstanding their superficial differences,
are said to be homologous because of the fundamental equival-
ency of structure.

in. Summary.

i. Division of labor and differentiation of structure proceed
together as the individual develops. All tissues retain the
power of using food, of oxidation, of eliminating useless prod-
ucts. Other functions incident?.! to these may be relegated to
special cells or tissues.

8o

ZOOLOGY.

2. Associations of tissues to accomplish a more or less defi-
nite work are called organs : organs of a similar kind are col-
lectively known as systems of organs.

3. The principal functions of animals and the organs or
systems performing the work may be classed as follows :

Function. System.

(a) Metabolism Nutritive.

(fe) Support and protection .. Skeletal, and integumentary.

(c) Growth

(d) Reproduction Reproductive.

(e} Motion Muscular, in connection with

skeletal.
(/) Sensation Nervous.

4. Metabolism or nutrition embraces the following proc-
esses :

(a) Ingestion of food (including oxygen),
(&) Digestion,

(c) Absorption,

(d) Circulation,

(e) Assimilation = anabolism,
(/) Dissimilation = katabolism,

(g) Secretion and excretion (of waste matter including car-
bon dioxid).

The processes in (a), (&), (c), and (e} are anabolic, i. e.,
add to the resources of the body. Those in (/) and (g) are
katabolic, i. e., tend to destroy the materials, develop energy,
and eliminate waste. Circulation contributes to the accom-
plishment of both purposes.

5. The supportive skeletal structures may be internal, or
external, or both. They may arise as a secretion of the super-
ficial cells of the body, or consist of a mixture of cells and
intercellular substance. Their nature and arrangement are
profoundly important in determining the distribution of the
other more active organs.

6. Growth and reproduction are the outcome of the nutri-

THE GENERAL ANIMAL FUNCTIONS. 8l

tive processes. Growth is increase in mass ; reproduction is
the production of new individuals from old. Reproduction
always involves cell division and may be asexual or sexual.
The latter normally involves two parents. In it two cells,
which may be either similar or dissimilar, must unite before
development will proceed.

7. The nervous and muscular systems are closely related
in function. Their united work is to receive, coordinate and
respond to the external or internal stimuli affecting the ani-
mal. The structures to receive stimuli (end organs) are
largely superficial; the coordinating and controlling parts
(central organ) are deep-seated, thereby securing protection
and a central position ; the muscular system must have definite
relations with the hard parts upon which it acts. Thus arises
the necessity of connectives or nerves between the various
portions.

8. The sense organs represent areas of the epithelium which
are peculiarly adapted to the reception of some one of the
forms of stimulus to which animals are subject, supplemented
by a more or less complex apparatus which serves to intensify
or modify the original stimulus. The sense organ determines
the kind of stimulus which may be received, but the central
nervous organ determines the nature of the sensation which
results, and the response.

9. Organs with different origin which by reason of similar
function have come to look alike are said to be analogous.
Organs with similar origin and structure, even though they
may appear differently because of their differing functions,
are homologous.

112. Topics for Investigation, in Library and Field.

1. What are the advantages and the disadvantages of divis-
ion of labor and differentiation? Illustrate your views very
fully.

2. Illustrate the variety of foods used by different animals
with which you are acquainted. Classify the animals you
know on the basis of their food preferences.

7

82 ZOOLOGY.

3. Compare the ways in which animals known to you cap-
ture and prepare their food for swallowing. What special
structures arise in connection with this function?

4. Do animals have any power of storing food within the
body for future use? Compare with plants.

5. Compare gills and lungs as to general form and arrange-
ment and see in what ways they appear to you to be suited to
their particular media, i. e., gills to water and lungs to the air.
Why might not the conditions be reversed?

6. What seem to you to be the comparative advantages and
disadvantages of the exoskeleton and endoskeleton.

7. Devices to accomplish locomotion in animals known to
the student. Find as many variations as possible.

8. Select four animals, as diverse as possible, representing
each of the following conditions of locomotion : through the
air, through the water, on the earth, and through the soil.
Compare the problems which each must solve, and the organs
by which the work is accomplished.

9. Compare known animals as to rate of locomotion. Do
you find a satisfactory explanation in any case?

10. Let the student attempt to prove that the dog experi-
ences the same sensations which we have. Hold him rigidly
to his evidence.

11. Report on the general differences between the eyes of
insects and of vertebrates, with a statement of their structure
and the work done by each.

12. In what way could the otocysts possibly act as organs
to enable the animal to appreciate its position in space, and
thus maintain its equilibrium?

13. What are the simpler facts connected with the process
of absorption or osmosis of dissolved substances in the body?

14. Find in text-books of chemistry a fuller account of the
process of oxidation and why it results in a liberation of
energy.

15. Demonstrate how a biconvex lens forms an image of
objects. Why inverted?

CHAPTER VII.

PROMORPHOLOGY.

113. We have seen in the preceding chapters how the work which an
organism must do is divided among its parts, and that the parts become
specialized in connection with this division of labor. This complexity
which is known as organisation is, in any animal, the result of forces
both within and without the animal, and expresses the adaptation of the
internal structures to each other and to external conditions. The simplest
organism known is thus organized. Organization is merely more evident
in the more complex organism. In addition to the fact of the organiza-
tion and heterogeneity of structure it is easy to see, after examining a
number of animals, that these different parts are not thrown together
without some definite order. For example, the ordinary vertebrates move
with their long axis horizontal, and possess certain organs that we always
expect to see at the anterior end ; their appendages are arranged in a
definite way in relation to the long axis. The parts of the starfish are
arranged according to a different but equally definite plan. All considera-
tion of the general plan according to which animals are constructed be-
longs to Promorphology. The fundamental plan may be similar in groups
of animals which are otherwise very different, because of similar external
conditions and similar modes of life.

114. Definition of Sections. In trying to express the plan of struc-
ture in animals it is convenient to have in mind certain planes to which
we can refer the parts. A section perpendicular to the main axis of an
organism or of an organ is called a transverse or cross section. The
longitudinal median section separating the body into right and left halves
is a sagittal section. A longitudinal section, perpendicular to the sagittal
and separating the dorsal (or back) and ventral (or belly) portions of the
body is described as a frontal section. An animal is said to be sym-
metrical with regard to any of these planes when the parts severed by
the plane are essentially similar.

115. Axiality. As an organism grows from its small beginnings in the
fertilized ovum, or from a spore in the simpler forms, the new materials
may be added more or less uniformly so that a mere increase in size
results ; or growth may take place more rapidly along some radii than
along others, making it depart from its original spherical fdrm; or mate-
rials or organs of one kind may occur along one radius and different ones
on another (as in Fig. 46). These lines of special growth and develop-
ment are called axes. We may investigate them as to their number, their
space-relations to each other, and the likeness or unlikeness of the two
ends or poles of each axis.

83

84 ZOOLOGY.

116. Types of Symmetry. It is desirable to distinguish the follow-
ing types :

I. In a spherical organism in which no differentiation is apparent (as
in a simple spherical cell, or blastula, Figs, n A, 3; 44) any plane passing
through the centre divides it into symmetrical parts and all the axes are
essentially equal. In such a case there may be said to be an infinite num-
ber of similar axes, and the poles of each axis are similar.

FIG. 44.

Fid. 44. Spherical cell (resting stage of Amoeba) illustrating general or universal
symmetry. Any plane passing through the centre will divide it into two essentially
equal portions.

Question on the figure. What prevents this animal being a perfect
illustration of universal symmetry?

FIG. 45. Amoeba in active condition. Entirely unsymmetrical.

2. An organism may be wholly asymmetrical, without any definite
form, the axes being without regular arrangement. {Amoeba in its active
stages, Fig. 45 ; some Sponges.) In other instances the form may be
definite and axes developed, but the structure is such that no plane will
separate the animal into symmetrical parts (Paramccium and many other
active Protozoa; see Figs. 66-69).

3. As a variation of the universally symmetrical condition seen in I,
a limited number of axes may become distinguishable from the others by
some specialization of structure (Fig. 46). These special axes are sim-
ilar and their two poles are alike.

4. Starting again from the undifferentiated spherical form, one of its
numerous similar axes may come to differ from all those perpendicular
to it by increased or diminished length, or by a difference in construction.
This special axis is to be known as the principal axis. The poles of the
principal axis do not usually remain alike. Perpendicular to this princi-
pal axis one may select an indefinite number of subordinate axes which
are essentially similar to one another. The poles of each subordinate
axis are alike. Such a condition is realized in the simplest gastrulae (Fig.

PROMORPHOLOGY.

II ; A 4, B 3, C 2). Any plane including the principal axis will divide such
an organism into two equal halves. In general external appearance a hen's
egg would illustrate the type. This is the least differentiated form of what
is known as radial symmetry.

FIG. 46.

P'

FIG. 46. Actinomma astcracanthion, a Radiolarian with a limited number of special-
ized radii (axes), symmetrically arranged about the centre. A, whole animal with
portion of two spheres of shell removed. B, section, showing relation of the proto-
plasm to the skeleton, n, nucleus; p, protoplasm; sk., skeleton. From Parker and
Haswell.

Questions on the figure. In what way does this species differ in
symmetry from Fig. 44? How many specially developed axes appear to be
present? By how many planes may the organism be divided into essen-
tially equal portions?

Two important variations from this simple condition of radial sym-
metry are found in the animal kingdom :

(a) Special organs, such as those of locomotion and the like, may be
developed about the principal axis. These usually come to be arranged
in a limited number of the planes which may be passed through the prin-
cipal axis. Considered from the point of view of the subordinate axes
this means that there are a limited number of special axes (Fig. 47, aa*
and bb l ) perpendicular to the principal axis (Fig. 47, o-ab.o) instead of
an indefinite number as in the former case. These special subordinate
axes are usually 3, 4, or 5, or some multiple of these numbers. The num-
ber however may be reduced to two in which the four poles are all alike.
Many of the medusae, coral polyps, and some echinoderms illustrate this
type of symmetry.

(&) A further variation of (a) is seen in the fact that in some ani-
mals, otherwise similar to those described in (a), one of these special
axes perpendicular to the principal axis comes to differ from the other.

ab. o.

FIG. 47. Diagram of Medusa, illustrating radial symmetry. A, viewed from the
oral end of the principal axis; B, a section along the principal axis and through one of
the subordinate axes ad 1 : o, ab. o, the oral and aboral poles of the principal axis; a, a 1 ,
and b, b 1 , the similar poles of the two chief subordinate axes.

Questions on the figure. Are the poles of the oral-aboral axis alike
or unlike? How many clearly differentiated secondary axes are there?
What would be the appearance of a section midway between ao 1 and && 1 ?
Would the resulting halves be symmetrical? Compare this condition with
the definition of radial symmetry in the text. Find other illustrations of
radial symmetry in the figures of this book.

The two poles of each of the subordinate axes are essentially alike, but
are unlike the poles of the other subordinate axis. This arrangement is
found in the sea-anemone (Fig. 48). The differences between aa 1 and
bb 1 are not usually so great that we cease to speak of the form as radially
symmetrical. It is of importance to know that in the radially symmetrical
animals the principal axis, whether longer or shorter than the subordinate
axes, is normally pc-rpendicular to the substratum on which the animal
rests, or to which it is attached.

5. If such an animal as was last described were to have its principal
axis horizontally placed, with one of its two subordinate axes vertical
and the other horizontal, and were maintained in this position, it would
likely happen that the formerly similar poles of the new vertical axis
would become unlike, because subjected to different influences. These
poles are known as the dorsal and ventral poles. The poles (right atid
left) of the other transverse or subordinate axis would remain similar,
as they are subjected in the long run to similar conditions. This gives
us the condition, found in all the higher, free-moving animals, known as
bilateral symmetry. It consists, to recapitulate, of (i) a main axis
(antero-posterior axis) usually horizontal and with dissimilar poles; (2)
a transverse axis, usually vertical (dorso-ventral axis) with dissimilar
poles ; and (3) a transverse axis perpendicular to the other two, hori-

PROMORPHOLOGY. o/

zontally placed, with poles alike (right-left axis). Such an animal has
only one plane (the sagittal) by which it may be divided into symmetrical
halves (Figs. 49, 29, 96).

117. Antimeres. There is a striking tendency among organisms to
repeat or duplicate organs or parts. This we have seen in the occurrence

FIG. 48.

B

ab. o.

FIG. 48. Diagram of the sea-anemone, illustrating another type of radial symmetry.
A, cross section; B, longitudinal section. Lettering as in Fig. 47. c, the chamber be-
tween the mesenteries (m).

Questions on the figure. How does the symmetry of the anemone
compare with that of the Medusa? Express the difference clearly in terms
of the axes and their poles. Are aa 1 and bb 1 strictly similar axes? Do
their planes divide the animal into halves? Are the four halves thus
obtained equivalent? In B what difference in the position of the section
will account for the differences on the right and left side of the figure?

of similar rays about the main axis, in radially symmetrical animals like
the starfish. Parts thus repeated are known as antimeres. The term is
also applied to the right and left or paired halves of bilaterally symmet-
rical animals.

118. Metameres. When the parts or organs are repeated in a linear
sequence along the main axis, as in the segments or rings of the Earth-
worm, the arrangement is described as scgmental or metameric. Metam-
erism may be shown both by the internal and external structures. The
vast majority of the elongated, bilaterally symmetrical animals are seg-

88

ZOOLOGY.

mented. In the higher Vertebrates it is not manifest externally, but is
shown in the vertebrae, the nerves, etc.

119. Appendages. Nearly all the animals, whatever the fundamental
symmetry may be, have appendages of one kind or another for locomo-
tion, capture of fodd, protection, respiration, and the like. These out-
growths from the body may be generally distributed over the body surface
(as cilia in some Protozoa and free-swimming larvae) ; or radially ar-
ranged, often about the mouth, as in many radially symmetrical animals
(Figs. 47, 48, 79) ; or in a right and left series in bilaterally sym-

FIG. 49.

,-Sp

FIG. 49. Diagram of the cross section of a fish, showing the bilateral symmetry of
the parts: dv, dorsoventral axis; rl, right-left axis. a. p., anterior appendage; b.c., body
cavity; ch, notochord; d.f., dorsal fin; g, gut; h, heart; h.a., haemal arch; m, muscles;
n.a., neural arch; sp, spinal cord; v.c., vertebral column.

Questions on the figure. In what respects is the symmetry as shown
in this cross-section different from that shown in the cross-section of sea-
anemone? Compare carefully, and express your conclusions in terms of
the axes and their poles. Find other figures in this book illustrating
bilateral symmetry.

PROMORPHOLOGY. 89

metrical animals (Figs. 129, 135, 188). The paired appendages of bilat-
eral animals may be attached dorsally, laterally, or ventrally, as deter-
mined by the uses they serve. They may be uniformly distributed along
the axis, one or more pairs to each metamere, as in some arthropods, or
be confined to special segments in certain regions of the body, as in the
higher arthropods and the vertebrates.

120. Practical Exercises. Let the student find illustrations, from the
animals with which he is acquainted, of paired appendages with dorsal
attachment ; with lateral ; with ventral. What is the work to be done by
each? Does their position appear to be of advantage in the performance
of it? Find likewise animals in which the appendages are clustered at
the anterior portion of the body; some in which only posterior appendages
are found, or at least are better developed than the anterior. Does the
arrangement seem in any way related to the habits and surroundings of
the animal?

121. Specialization of Metameres. In the lowest segmented animals,
as worms, the metameres are much alike in external form, in their ap-
pendages, and in the contained structures. In the adult insects this be-
comes less true, and the various segments are specialized for particular
duties. The segments in the head region become very different from the
body segments. The same is even more true of the higher vertebrates.
This progressive differentiation of a distinct head is one of the most
remarkable facts to be noted in animal development. Accompanying the
specialization of groups of segments in various parts of the body we often
see the complete fusion of such similar segments for the better perform-
ance of their common work (as in the head of insects and vertebrates,
the thorax in insects and Crustacea).

122. Formation of New Segments and Regeneration. In many of
the animals in which the segments seem of nearly equal value there is a
more or less continuous formation of new segments. By this process the
organism increases in length and in the number of its segments, and
frequently, with the aid of a somewhat similar process, produces two
individuals by division, as in many worms. Such a proceeding necessi-
tates the formation of a new head or tail in each of the daughters, by a
segment which, in the mother, was a body segment. When such an animal
is artificially cut in two, each half may reproduce segments like those
which have been removed from it. This is known as regeneration. Nat-
urally this ceases to be possible in animals in which the segments become
more highly specialized ; yet even in the highest animals some power of
replacing lost tissue or even lost organs remains (as in healing of wounds,
formation of a new tail by lizards, etc.). It is recognized as a general
law that in making these repairs or healings the newly-formed material
tends to restore the symmetry possessed at the outset.

123. Summary.

I. Promorphology treats of the ground plan in accordance with which
the parts of animals are combined.

90 ZOOLOGY.

2. Symmetry relates to the possibility of passing one or more planes
through the animal and obtaining similar portions on either side of these
planes.

If no such division is possible the organism is described as without
symmetry. If each of three mutually perpendicular planes separates the
animal into equivalent portions, it may be described as universally sym-
metrical. If no plane transverse to the main axis can divide the animal
into symmetrical parts, and two or more, which split the animal along
the main axis, are capable of doing so, we describe the form as radially
symmetrical. When there is only one such plane capable of separating
the body into equal parts we have the condition of bilateral symmetry,
which represents the highest condition of development, that of active
animals.

4. Antimeres are parts of animals repeated on different sides (two or
more) of the main axis of the body.

5. Metameres are parts repeated in the main axis, i. e., one behind
another. The successive metameres may be almost entirely alike (homon-
omous), or they may become much differentiated in the performance of
diverse functions (hcteronomous).

6. From the main trunk of animals special appendages often appear.
They usually adapt themselves to, and accentuate, the fundamental sym-
metry of the organism. They may therefore be asymmetrically placed,
or uniformly distributed over the entire surface, or along the radii, or in
pairs as in the bilaterally symmetrical forms. There are typically one or
more pairs to each metamere, though this number may be much reduced.
Paired appendages, in series, are regarded as homologous.

7. Many animals have the power of restoring by growth parts or seg-
ments which have been lost (regeneration). In the lower segmented
forms this power is closely associated with the power of increasing the
number of new segments in an uninjured animal. In heteronomously
segmented animals both these powers are less manifest.

124. Topics for Investigation.

1. Determine the nature and degree of symmetry in (i) the sponge
of commerce; (2) skeleton of starfish; (3) crayfish or grasshopper.

2. What is the final criterion by which you determine which is the
anterior and which the posterior end of an animal? Justify.

3. Find among animals of your acquaintance instances of difference
between the dorsal and ventral surfaces as to color, form, etc., and see
if you can discover any possible advantage resulting therefrom.

4. What degree of difference have you ever noticed between the right
and left halves of the body in various animals? Is perfect bilateral sym-
metry ever found?

5. Can you assign any reason for the location of the sense organs at
the anterior end of the body? (Distinguish between cause and advan-
tage.) Do you think they occur here because it is anterior, or is it ante-
rior because they occur here?

PROMORPHOLOGY. pi

6. Why are vehicles (made by man) bilaterally symmetrical?

7. Express in tabular form the differences between the poles of the
three axes of bilateral animals, and what you can gather as to the causes
of the differences.

8. For what kind of life does bilateral symmetry fit the animal?

9. For what kind of life does radial symmetry seem suited? Verify
by illustrations.

10. To what extent do animals seem able to regenerate lost parts?
Trace out the conditions in various groups, as far as your reference library
will allow.

11. Is there any apparent limit to the numbers of metameres which
animals may possess? What degree of constancy or variability in this
particular can you discover in various individuals of the same species?

CHAPTER VIII.
INDIVIDUAL DIFFERENTIATION AND ADAPTATION.

125. The Individual and its Environment. We have
thus far considered the mature individual as the end for which
the various developmental processes exist. It has been seen
that the individual becomes complex as its parts grow and
become differentiated to do the work necessary to the well-
being of the animal. In this differentiation the parts become
dependent upon each other, and in the healthy state they work
harmoniously among themselves. It is now necessary to pass
from the consideration of these internal structures and rela-
tions in order to consider the individual animal as a unit in its
relations to everything about it, that is, to its environment. This
term includes not merely the inanimate materials and condi-
tions surrounding an organism, but in addition all living things
both plant and animal which directly or indirectly influence it.
The environment of no two animals is the same, nor is it the
same for any given animal for two moments in succession.
This continual change in the environment leaves its impress
on the structure and habits of all organisms. Every individual
is thus related to its own environment from day to day; in
addition to this, all the individuals of any generation, owing
to the facts of reproduction, are also to be considered in the
light of the conditions to which their parents have been sub-
jected from the remotest time. The study of the individual in
its relations to its environment brings the student face to face
with many very important problems. No department of
zoology is more interesting.

126. Heredity. One does not study organisms very long
without being impressed with two things : first, that there are
remarkable similarities among them, even among those little

92

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 93^

related; and second, that there are interesting differences
among even those whose kinship would entitle them to great
likeness. There is always a disposition among students to
feel that the likenesses are due to internal causes and that the
unlikenesses are due in some way to the varying external in-
fluences. In other words the former are thought to be due
to heredity and the latter to the environment.

Characteristics which animals receive from their ancestors
near or remote are described as hereditary. We ascribe the fact
that the hen's egg produces a fowl and a frog's egg, a frog to
the action of heredity. No less is the repetition in the child
of minute parental peculiarities of feature and form a fact
of inheritance. While these likenesses are due to the action
of the internal forces of heredity, it must not be deemed that
heredity is a purely conservative influence in the life of the
organism. The offspring of two parents may inherit entirely
different qualities from their parents and thus present differ-
ences among themselves due solely to inheritance. The off-
spring may also present such a mingling of the qualities in-
herited from their ancestors as to possess characteristics
decidedly new to any of them. Thus likeness to parents and
unlikeness to parents may equally be due to heredity. It at
once perpetuates old qualities and introduces variations.

It was formerly considered that all the characteristics which
parents possess are equally subject to inheritance, but it is
now denied by many biologists that the qualities which a par-
ent acquires in its own lifetime, as the result of its own actions
or of the environment, are capable of being transmitted. It
is unquestioned however that qualities received from the par-
ents are, under favoring circumstances, capable of being trans-
mitted to offspring.

127. The Bearers of Heredity. It follows from the fact
that the adult organism is produced from the union of the male
and female elements that these two cells are in some way en-
dowed to carry the parental qualities. There are strong evi-
dences that the chromatic elements, or chromosomes, in the

94 ZOOLOGY.

nuclei of the male and female cells are the material structures
by means of which transmission is effected. The chromosomes
of the fertilized ovum are contributed equally by the male and
female elements, and they are the only structures in the sperm
and ovum which are apparently equal in amount. This taken
in connection with the fact that one parent does not have any
more power, on the average, to influence offspring than the
other furnishes a basis for the belief that the chromosomes are
the physical basis of heredity. Recent investigations tend to
show that the male and female chromosomes retain their dis-
tinctness and are equally distributed to all the nuclei of the
developing embryo.

128. Library Exercises. The student may increase his knowledge of
the facts of heredity by endeavoring to find answers to the following ques-
tions. What is atavism, and what explanations have been offered for it?
Do the male and female seem, as a rule, to have equal power of trans-
mitting their individual characteristics? Cite some facts tending to show
that the nucleus is especially concerned in transmitting parental qualities ;
that the chromosomes are instrumental therein. What are the essential
features of the old " preformation " hypothesis to account for the fact
that an adult similar to the parent springs from an egg? Examine some
of the principal theories of inheritance : Darwin's " pangenesis " ; Brooks'
modification of it ; Weismann's " continuity of germ-plasm," etc. What
is Mendel's law of inheritance?

129. Variability. Notwithstanding the fundamental like-
ness existing between parent and offspring, and among the off-
spring of common parents, no two individuals even among the
lowest animals are exactly alike. This fact of variation is only
less fundamental than the fact of likeness. Variation among
animals appears to depend upon two sets of considerations:
( i ) the physical and chemical instability of the protoplasm of
which animals are so largely, composed, and (2) the diversity
of the environment in the broadest sense. Through the inter-
action of these two influences, even if all individuals were alike
at the start, it would only be a question of time until the off-
spring derived from them would present noteworthy differ-
ences. Such differences would tend to increase with the lapse
of time. This is the more true in proportion to the degree in

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 95

which variations are capable of being transmitted under the
influence of heredity. There is no known limit to the power
of organisms to vary.

130. The Part Played by the Environment in Producing
Variation while not completely understood must be recognized
as very real. Even though much stress must be put upon the
hereditary complexity and instability of protoplasm as the
source of variations, it is evident that the external conditions
serve as stimuli to produce the changes on the inside. For
example, it is a matter of common observation that the quan-
tity and quality of food greatly influence not merely the rate
of growth but the size and quality of the adult organism as
well. Life would be impossible without food, oxygen, water
and suitable temperature. Any variation in these conditions at
once has its effect upon the organism. Experiment shows that
the varying degrees of salinity of the water may be accom-
panied by striking individual differences of form in certain
marine animals. Caterpillars of certain butterflies placed in
boxes lined with differently colored papers develop pupae with
colors harmonizing with those of the boxes containing them.
Colors in various animals are intensified or changed by special
foods or by changed temperature. In general it may be said
that changes in any of the conditions important to animal life
produce some change or variation in those animals subjected
thereto. Since this is true, it becomes inevitable that the
various individual animals on the earth are differentiated from
each other somewhat as was seen to be the case with the cells
and tissue of which the individual itself is composed. The
following paragraphs trace out some of the ways in which
this differentiation of individuals takes place, the relations of
the various organisms to each other and to the environment.

131. The Struggle for Existence. All animals (with a
few possible exceptions in those which possess chlorophyll)
depend ultimately upon green plants for food, those which
live on other animals no less than those which use plant food

96 ZOOLOGY.

directly. Only a limited amount of vegetation can. be sup-
ported by the earth without cultivation. The number of ani-
mals therefore which can find a livelihood on the earth is in
turn restricted. There is, however, no such limit of the powers
of reproduction, either among plants or animals. Any pair
of organisms if unchecked could in a very few years supply
descendants enough to populate the earth up to its full powers
of support. That they do not thus multiply at a geometric
ratio is due solely to the influences at work to destroy these
descendants. Any group of organisms will hold its own when,
on an average, a pair of individuals can in a life time bring to
maturity another pair to take their place. More than this
means conquest of new territory ; less than this, the extinction
of the group. When we recall that all organisms have this
unlimited power of reproduction, it is easy to see that a time
must soon come when a struggle for food and a foothold on
the earth is inevitable. The struggle would be more intense
between those organisms which demand the same kind of food,
that is, among kindred. This is the fundamental struggle. It
would be complicated by the fact that some groups of animals
prey upon others, and that the primary conditions of life, as
water, temperature, etc., are subject to striking changes.
These facts tend, by just so much as they destroy individuals,
to relieve the struggle within the species, and to introduce new
factors which give great variety and interest to the life prob-
lems of animals. There is nothing more certain than that this
struggle has occupied organisms practically from the begin-
ning, and all our explanations of present conditions must take
note of the fact. All the important structures and activities
of animals are modified by this competition for a livelihood.

132. Library Exercises. The student should be invited to make real
to himself the possibilities of a geometrical increase as applied to organ-
isms. Take the known rate of increase (that is, the total number of
descendants in an average lifetime) of a number of common animals and
determine the possible living descendants in a .specified time. Find ref-
erences concerning infusoria, insects, fish, man. Have you any observa-
tions relating to the reality of the struggle for food among animals?

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 97

133. Natural Selection. In spite of this power of repro-
duction we see that, on the average, individuals do not increase.
The earth is no more thickly inhabited by animals today than
it has been for countless ages. The proportions of different
animals vary now and again, but that is all. Out of a family
of one hundred young individuals, no two of which are alike,
striving for a foothold, ninety-eight will be destroyed. Which
will survive? Barring accidents beyond the powers of any of
the individuals to resist, those will survive which possess or
acquire some quality, structure, or habit, suited to the struggle
in which they find themselves. This may be a matter of
strength, of speed in eluding enemies or capturing prey, of
specially acute senses, of a tendency toward concealment, or
any one of a thousand things calculated to fit an organism for
a special place in life. It is not necessary to suppose that these
elements of fitness exist in striking degree at first. The strug-
gle is so intense that even the slightest handicap may mean
the destruction of the individual. This elimination of the
weaker individuals results in what has been called natural
selection through the " survival of the fittest." The hereditary
qualities thus preserved in the individual are subject to trans-
mission by heredity; and by the continuous action of natural
selection and heredity through a long series of generations
these elements of fitness are believed to accumulate, and thus
animals become better and better adapted to their surroundings.

134. Artificial Selection. Since man has been on the earth
he has been a most potent factor in the environment of the
other animals. He has helped in the elimination of animals
hurtful to his interests; has domesticated others which he has
deemed useful, thus rendering their environment highly arti-
ficial and removing from them the struggle for existence in
certain measure. For natural selection he has substituted a
conscious selection of such organisms as are best suited to his
needs or fancies, and has allowed these to reproduce, eliminat-
ing the others. This artificial process, which obtains results

9& ZOOLOGY.

more rapidly than the natural, has given rise to the various
breeds, strains or races of dogs, horses, cattle, fowls, etc.
By means of this selection the habits and dispositions of the
domestic animals have been improved as surely as their struc-
ture. Their power of self-support, however, has been so ma-
terially diminished that some of them could not succeed in
finding a living in the wild state under ordinary circumstances.

135. Practical Exercises. Are there any domesticated animals whose
species is represented in the wild state? Compare the habits and general
structure of some of the domesticated animals with that of their nearest
kin among wild species. How many species of domestic animals can you
enumerate? From what groups do they come? Trace the history and
results of the domestication of some of the common animals, as fowls,
pigeons, cats, dogs, etc. Have any strictly American species been
domesticated?

136. The Adaptation of Animals to their Environment.
There are two distinct questions of importance to be con-
sidered in connection with this subject: (i) the necessity of
the adjustment of organisms to their environment, and (2) the
means by which this adaptation takes place in the individual
and becomes fixed in the species. It is clear that the limited
food supply and the unlimited powers of animals to reproduce
result in a struggle for food among the animals at any time
occupying the earth ( 132) . This struggle is not merely among
the animals in question, but is in reality between every organ-
ism and its whole environment. Extremes of heat and cold,
drouth and famine, and numerous changes in the conditions
of life make it absolutely necessary that the individual shall
have some power of adapting itself to what is permanent and
what is changeable in its environment. What are the means
then by which animals that are not completely in accord with
their surroundings may become so? There are two possible
ways in which this may come about. The animals may mi-
grate to regions where the conditions are naturally more favor-
able to their well-being, that is to regions for which they are
already adapted. As a matter of fact this is known to be a
common occurrence. Animals often disperse from their old

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 99

centre of multiplication under the influence of hunger or un-
favorable local conditions. They are often assisted in these
dispersals by such natural agencies as winds, currents of water,
and by other animals. If the migrating forms succeed in find-
ing new regions suited to their needs, there results a condition
of adaptation between organisms and their respective environ-
ments, but without any active change in the characteristics of
the organism. The environment itself is subject to continual
change and there are too many barriers in the way of universal
migration for this to be accepted as a complete explanation of
the widely observed adjustment of animals to the conditions
which surround them.

In the second place animals may become suited to their en-
vironment by variation, without migration. There is no ques-
tion that this also occurs and that it is the more important
factor of the two. It has been shown (129) that all animals
are variable. Students of biology have suggested two impor-
tant ways in which variations may give rise to a harmony
between the organism and its surroundings. This result may
take place through natural selection (133). According to this
view the organisms naturally tend to vary. The changing en-
vironment stimulates this tendency to variation. Out of a
thousand individuals of similar parentage there will be numer-
ous slight differences of structure and physiological qualities.
Some of these will be more, and some less, favorable to the
environment. In the struggle those will be eliminated which
for any reason are strikingly unsuited to the environment. On
the other hand those animals whose variations are most in
accordance with the local condition will persist and propagate
their kind, tending through heredity to pass on to their off-
spring the qualities which enabled them to adjust themselves
to their surroundings. Thus there will be a gradual, ever
increasing adaptation in the whole species of which they are a
part, by natural selection. Occasionally there occurs in off-
spring, from the action of the environment or from other
causes, a sudden and considerable change from the parent

100 ZOOLOGY.

type. Such a product is known as a " sport." It is quite pos-
sible that natural selection may seize on such and if in a favor-
able direction preserve and increase them. In such cases adap-
tation might take place with great rapidity, instead of in the
gradual way described above.

As contrasted with natural selection of indefinite variations,
it has been argued that the immediate effect of the environment
on the organism and the efforts of the organism to respond to
the stimuli of the environment produce in the organism just
such definite variations as will tend to fit it for its surroundings.
In other words the majority of the variations brought about
by a given external condition are definite and naturally in the
direction to meet the necessities of the case. For example, cold
stimulates the surface cells of the body of an animal. The
immediate response of the nervous and nutritive processes in
the organism are such that the surface cells take on greater
activity and produce materials at the surface of the body
which tend to protect the animal from the ill effects of the cold.
This is an individual variation. To become effective in making
the species better adapted to the environment these results
must be handed down by inheritance to the next generation.
If this can take place this theory would go a long way toward
explaining how adaptations arise. There is, however, con-
siderable doubt whether such adaptations acquired in the life
of an individual can be transmitted to offspring. If this cannot
occur we are thrown back upon natural selection as the prin-
cipal explanation thus far offered to account for the pro-
gressive adaptation of animals to the environment. There is
no reasonable doubt that natural selection is such an explana-
tion. To what extent it is assisted by other factors is at pres-
ent uncertain. It will be assumed in the following pages that
it is the most important known factor in producing adaptation.

137. Classification. Since the environment is not the same
at any two places on the earth and there is an accumulation,
from generation to generation in animals, of those features
which tend to bring them into harmony with their different

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. IOI

environments, it is inevitable that the animals themselves come
to be very diverse, no matter how similar they were at the
outset. In the discussion of them it therefore becomes neces-
sary to devise some means of expressing the degree of like-
ness and unlikeness among the great number of individual
animals existing on the earth. This may be done by means of
an appropriate classification. The differences of structure and
function may be superficial or fundamental, but it must be re-
membered that all these differences are in some way the out-
come of the history of the organisms, and that the likenesses
are signs of kinship, or of similar history, or both. The group-
ing or classifying of organisms has two objects: (i) con-
venience, that is, to make future work easy; and (2) to express
the results of past study. Insomuch as the first motive may
predominate the classification may be artificial, that is may
bring together animals that are really not closely related,
though possessing a superficial resemblance. The grouping
together of bats and birds on the ground of their power of
flying, or whales with fishes because of their habitat, would
illustrate such a classification. In proportion as classification
takes in all the facts known with regard to animals and ex-
presses the relationship of forms classed together, it is said
to be natural. Every classification is in some measure artificial
since we do not know all the facts concerning the structure or
history of any organism.

138. Terms Used in Classification. From what has been
said concerning the power of multiplication in animals, the re-
sulting struggle for existence, the variability, and the elimina-
tion of those whose variations are not suited to the various
environments into which the offspring migrate, it will be
readily understood that even the descendants of a single pair
of organisms will come in time to be noticeably different in
form, size, color, and the like. The individuals of a given
region will usually be more like each other than like their
cousins who have been subjected to some other kind of en-

102 ZOOLOGY.

vironment. There is thus a need of terms to express the de-
gree of difference which, through these influences, finally char-
acterizes the descendants even of common ancestors. Such
groups of forms are usually known as varieties or subspecies
of the original type from which they all sprang. Thus in the
human -race while all men are considered as belonging to one
common type and possibly derived from the same human
ancestors there is enough difference between the American
Indian and the Caucasian to make it necessary to distinguish
them as different varieties. Many of our widely distributed
animals as the dog, the horse, the common fox have varieties
which are readily distinguishable. When the causes which
produce varieties have been at work long enough to eliminate
the intermediate forms which are often found connecting the
varieties, and to secure a close adaptation of the varieties to
the environment, the term species is applied to what were for-
merly called varieties. Species thus merely represent the fur-
ther progress 1 of individual differentiation and adaptation to
the different modes of life which give rise to variation in
individuals that is, to varieties. A species of animals may
again split up by the action of the forces mentioned (and other
conditions which have not been mentioned) into new varieties
and finally into new species. It is believed that the present
diversity of animal and plant life has come about from a much
more limited number of kinds of ancestors by a method essen-
tially such as that described above. Varieties of the same
species usually cross freely and the offspring are known as
mongrels. The individuals of different species as a rule cross
less freely and when they do cross their offspring are called
hybrids. Hybrids are often sexually infertile.

The genus is related to species somewhat as the species to
the varieties which compose it. A genus embraces those kin-
dred species which show a high degree of relationship among
themselves. The characters which serve to distinguish differ-
ent genera are more fundamental than those by which we
recognize varieties or species, and argue a more extended time
in the differentiation of genera than is required for species

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 103

Other terms, as families, orders, classes, phyla, are used to
denote the still more extensive and comprehensive divisions
of the animal kingdom.

139. Illustration of Classification. The domestic cat has
many varieties or breeds, as the maltese, manx, tortoise-shell,
etc. On the other hand, the wild-cat, the tiger, the leopard,
the lion have numerous points of -structural likeness to the
domestic cat, and are said to be species belonging to the same
genus (Felis}. The genus Felis and others less common are
placed together in the family Felidce. These with the members
of the dog family and others constitute the order Carnivora
(flesh eaters), and similarly for the higher groups in the
diagram below.

Kingdom Animalia.

Phylum Chordata (fishes, birds, mammals).

Class Mammalia (carnivora, ruminants, bats, man).
Order Carnivora (dogs, wolves, cats, etc.).
Family Felidse (cat family).

Genus Felis (cat, lion, tiger, etc.).

Species Felis domestica (with its numerous

varieties).

The name of an animal is its generic name followed by its
specific name as above. The variety name is added when there
are distinct varieties.

140. Relation of the Individual to the Species. The vari-
ous types of animals produce their offspring in numbers pro-
portional to the difficulties encountered in bringing the young
to maturity. In the most favorable circumstances many more
are produced than can survive. In cases where enemies are
numerous millions of eggs may be deposited in order to secure
a single adult. Nature is thus said to be lavish in her waste
of individuals in order that the species may be continued and
improved in its adaptations. These surviving descendants,
generation after generation, have become, through natural
selection, more and more suited to their surroundings. This

104 ZOOLOGY.

means that the production of many individuals, a large num-
ber of which never reach maturity, secures the development of
a small aristocracy which propagates the type. The species
is related to its individuals something as the individual is to
the renewed and changing cells of which it is composed.
Species are not constant, but even the most fixed must undergo
change or extinction when confronted by new conditions.
Species however are less variable than the individuals com-
posing them because the species represents an average con-
dition of all its individuals. Adaptation to environment is the
great problem which every animal must solve. Those which
do solve it successfully constitute the species. It is needful
then to consider next those characteristics of structure, habit,
or instinct whereby a species of organisms becomes success-
fully adjusted to its surroundings. In a broad sense all the
organs which were outlined in the preceding chapters are adap-
tations : the digestive organ and process, to the nature of food ;
the nervous system and the special senses, to the external
stimuli; the lungs, gills, and skin to the need of oxygen, and
the like. In contrast to adaptations of this kind we now con-
sider as adaptations those more special modifications of fun-
damental structure by which a species becomes more suited
to some limited habitat or to some special mode of life which
is of signal use to it in the struggle for existence.

141. Classification of the Principal Types of Adaptation.

A. Adaptations primarily in relation to the inorganic en-
vironment.

B. Adaptations primarily related to other organisms.
I. Among animals of the same species.

1. Friendly and social,
(a) Mating.

(&) Parental care of young.

(c) Organic colonies.

(d) Social and communal life.

2. Competitive : for food, mates, etc.

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. IO5

II. Among animals of different species.

1. Friendly and social,
(a) Commensalism.
(6) Symbiosis.

2. Competitive.

(c) The predaceous habit: adaptations for offense
and defense.

(d) Parasitism.

142. Adaptations to the Inorganic Environment. These
embrace such special structural devices as hair, feathers, the
blubber of whales, which enable the body to maintain its tem-
perature despite the condition of the medium. The habits of
burrowing and hibernation, the winter migrations of many
animals, especially birds, are examples of instinctive adapta-
tion to cold. The same end is obtained by man by artificial
clothing, by houses, and by the use of fire which has been one
of the most important instruments in his progress. Rotifers,
infusoria, and some other animals have become capable of
retaining life during thorough drying, and of resuming activ-
ity on the return of moisture. Adaptations to locomotion in
different media, earth, air, and water; to climbing; to station-
ary life, belong to this group. These are only a few of the many
instances of adaptations of organisms to the materials and
the forces about them. It is easy to see that some of the adap-
tations are of life and death importance, and without them the
species would become extinct. It is believed that these qualities
of the organism arise in a way something like this : owing to
the irritability of all protoplasm, the prevalent external factors
as heat, light, gravity, moisture, and chemically active sub-
stances must produce some change, that is, some response
on the part of the organism. Those organisms in which the
response is not in accordance with the best adjustment to the
special environment are less likely to survive in the struggle
for existence. Those which do survive and propagate their
kind because of their favorable responses to these stimuli are,

io6

ZOOLOGY.

by reason of these facts, more and more likely in succeeding
generations to possess those habits and structures suiting them

to their surroundings.

FIG. 50.

FIG. 50. Young Opossum (Didelphys rirginiana) photographed by J. W. Folsom.

Questions on the figure. Of what conceivable value to the animal
is the prehensile tail? In what other groups of animals is the tail pre-
hensile? What are the habits of the opossum? How is this species dis-
tributed on the earth? Where are other marsupials found?

143. Practical Exercises. Find other instances which seem to indicate
adaptation either in structure or habit to special features in the environ-
ment : as adaptations to prevent undue evaporation in a dry climate ;
adaptations to warm conditions ; to drouth ; to the use of special plants
as food ; to light ; to gravity. Illustrate from observation and by library
reference the types of adaptations cited in the text above. Is the power
of sleeping an adaptation of any value ? Among what animals is it found ?
Find instances in which useful adaptations have become useless and even
hurtful from changed conditions of life.

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. IO7

144. The relations of animals of the same species to one
another is an interesting mixture of competition and coopera-
tion. In the higher forms the parents instinctively make great
personal sacrifices that the offspring may be cared for; the
offspring on the other hand struggle with each other for this
parental provision. In the classification offered (141) it
should be remembered that both friendly and competitive habit?
and structures are always represented in the same individual.

145. Mating Adaptations. One of the most striking
forms of individual variation is seen in the differences be-
tween the sexes of higher animals. The male and female are
often so widely different in form, size, color, and other quali-
ties, that naturalists have classified them as belonging to dif-
ferent species and yet it is very manifest that, though different,
the sexes are closely adapted to each other. In the lower types
of animals the sexes are frequently represented in the same
individual. In such cases the sexual elements often mature at
different times. An individual is thus alternately male and fe-
male. This is regarded by many as being the primitive condi-
tion, the separation of the sexes being accomplished by the
repression, so to speak, of one or the other sex in each individual.
It is now known that the temperature and the amount and qual-
ity of food have something to do with the proportion of males
and females which are produced. So sexual dimorphism is
in some measure a response to external conditions and presents
every evidence of being an advantageous adaptation to the
conditions of life. The very union of the sperm and the ovum,
whereby two cells lose their individuality in one, with a renewal
of powers and the mingling of the qualities of two parents,
must be looked on as an adaptation of the very highest moment
to the animals in which it first appeared, and to their de-
scendants. The chemical attraction which the female cell
exerts on the motile sperm cell is a special adaptation to
accomplish this union. Furthermore it is undoubtedly true
that many of the color-markings, notes, motions, and the like

108 ZOOLOGY.

in which the male and female animals differ are recognition
marks whereby the presence of one sex is made known to
the other. In some animals in which fertilization normally
occurs, the ova may develop in the absence of sperm (parthe-
nogenesis). This may have arisen as an adaptation to tem-
porary scarcity of males. This view is in some cases supported
by the additional fact that parthenogenetic eggs produce male
individuals wholly, or in excess.

146. Practical Exercises. What is the difference in the notes of the
male and female of the American quail which would serve as recognition
marks? Mention other cases of sexual dimorphism which appear to you
to serve a similar end. What evidences have we that the mingling of
sperm and ovum results in a rejuvenescence? in the introduction of greater
variation? in conservatism? Show how these are important as adapta-
tions in the struggle for existence. In what groups is parthenogenesis
found? Give details of the facts in several cases.

147. Reproduction and Care of Young. The very rate
of reproduction is an adaptation to the severity of the strug-
gle for existence experienced by the animals of a given species.
Those forms with few enemies and abundant food usually
need to produce only a few young in order to maintain their
place. Others less favored in these regards, as many insects,
the lobster, the salmon, must reproduce thousands of young
in a lifetime. Similarly the length of the reproductive period
and of life becomes an adaptation to the same end. It is clear
from these facts that any device which the parent may adopt
likely to bring a larger percentage of the young to maturity
will make for a saving in the necessary birthrate. This hus-
bands the parental resources and conduces to the efficiency of
the individual and of the species. It must not be supposed that
parental care is confined to the higher animals. In its most
elementary condition it takes the form of food stored in the
egg, and in depositing the egg in a safe place for hatching.
After hatching it takes the form of supplying food, or protec-
tion, or both. Cephalopods, fishes, and birds have a large
amount of food substance stored in the egg. Many animals,
as the clam, some fishes, some reptiles, and the mammals, re-

INDIVIDUAL DIFFERENTIATION AND ADAPTATION.

FIG. 51.

FIG. 51. Galls on oak, cynipid (Holcaspis duricoria). Natural size. Photo by Folsom.

FIG. 52.

FIG. 52. Galls on elm, produced by an aphid, Colopha ulmicola. Natural size.
Photo by J. W. Folsom.

no

ZOOLOGY.

tain the eggs in special portions of the body until development
has well begun. The flies lay their eggs in the decaying matter
which the young use as food. The solitary wasps seal theirs
up in nests with the food (dead or wounded spiders or insects)
on which they are to develop. Other insects bore into the
tissues of living plants and deposit their eggs, about which
" galls " or masses of abnormal vegetable tissue are developed.
The ichneumon fly deposits its eggs in the body of some other

FIG. 53.

FIG. 53. Galls on hackberry leaf, produced by a fly (Cecidomyiida). Natural size.

Photo by Folsom.

Questions on figures 51, 52, 53. What does the gall represent from
the point of view of the plant? From the point of view of the insect?
What seems to cause the undue vegetable growth? Find other galls in
nature and try to find what type of insect is responsible for them? In
what ways may one hope to determine this fact?

animal. Thus we see an immense number of adaptations
useful to the organism have been developed in connection with
the egg-laying habit. After such provision the majority of
animals leave the young to care for themselves; but many
higher forms take further pains to protect and train their
offspring during the course of their development. The care
which the birds and mammals give their young is a matter
of common observation. It takes the form of food, of special
homes, as nests, burrows, dens, etc., and of the personal ser-

INDIVIDUAL DIFFERENTIATION AND ADAPTATION.

Ill

vices of the parents, who will often at the risk of their own
life protect the young from its enemies. Similar care is shown
by some insects, especially the social forms, such as bees, ants,
and the like. The lobster carries its young on its abdominal
appendages for months after hatching. The lower Inverte-
brates are practically destitute of these later care-taking
instincts.

It is interesting to notice that animals differ very much in
their helplessness at hatching or at birth. The young of the

FIG. 54.

FIG. 54. Nestling Marsh Hawks (Circus cyaneus). From Year-Book. Department of

Agriculture.

Questions on the figure. What are the nesting and breeding habits
of the marsh hawk? Are the young precocial or altricial?

reptiles, or the duck, or the chicken are relatively well devel-
oped at hatching, and are very soon able to run about and
feed. The young of the song birds, as the thrushes, swallows,
etc., are wholly dependent on the care of the parents for a
considerable ti'me. In the herbivorous mammals, as the sheep
and cattle, the young have the use of their limbs in a short

I I 2 ZOOLOGY.

time. Among the carnivorous forms, as the cat and dog, the
young are more helpless. In the human species the period
of helplessness is longest and consequently the necessity of
parental care greatest. In general, the longer period of
parental protection accompanies the development of more com-
plex and highly organized instincts, and intelligence. The
lengthened period of dependence, while a burden to the parent
in one sense, is an advantage to it in the saving in number of
offspring, and serves to benefit the species, not merely by keep-
ing the offspring alive until they may reproduce, but in the
greater development of such parental instincts as gentleness,
self-sacrifice, and the like. In the human race it has given rise
to the home and family, which we regard as the real basis of
modern society; and social organization in turn has been a
most powerful factor in the progress of the human species.
Death and the length of life must also be considered as special
adaptations. This differs in different species very widely.
Life in general, where natural selection acts, will be the period
of youth, plus the period of fertility, plus the time necessary
to rear the latest offspring. For the species, the death of the
individual becomes an advantage at the completion of this
period, and this fact is sufficient to insure that death will
normally occur at this time.

148. Practical Exercises. Add instances of parental care which have
fallen under your own observation, and give a statement of the facts in
the case. Compare the mammals with which you are acquainted in this
regard. Compare the condition of the young of the robin, the quail, the
blue-jay, the pigeon as to maturity at hatching. Do any animals of your
acquaintance reproduce more than once in a year? Why is one reproduc-
tive period per year a common adaptation. Compile statistics concerning
the longevity of various animals, and its relation to size, to reproductive
period, and to the time demanded to reach the adult stage.

149. Colonies. In some of the lower groups of animals,
as the polyps and jelly-fishes, in which the reproduction by
fission or budding is prominent, the newly-formed individuals
remain for a longer or shorter time in association with the
parent, or with each other. These units which otherwise might

INDIVIDUAL DIFFERENTIATION AND ADAPTATION 113

be separate individuals are organically connected and often
come, by the continuation of the process, to form immense
masses, as in the coral. Such organic associations are called
colonies. Colonies rarely occur in animals in which the organs
are highly specialized. Very often the individuals become
specialized for the performance of a special portion of the
work, and thus we get several quite differently constructed
individuals within the colony (polymorphism, Fig. 85). The
whole colony may then behave somewhat as an individual, the
polyps taking the place of organs (Siphonophora). Colonial
animals are almost always attached to fixed or floating objects.
These polymorphic individuals are closely adapted to each
other in structure and division of labor ; and the colonial habit
in general, even where there is no division of labor, is a suc-
cessful device whereby limited areas are completely occupied
by the members of a species (as in the case of the branching
corals) where the single polyps would be practically helpless.
The arrangement of the polyps on the common skeleton and
the rate of growth of the different polyps are beautifully
adapted to the best use of the currents of water by which the
food and oxygen are conveyed.

150. Library Exercise. What phyla of the animal kingdom supply
instances of organic colonies? Trace different degrees of polymorphism.
In what different ways do the individuals occur on the common stock?
Show how the relative rate of growth of the differently placed individuals
determines the ultimate form of the colony as a whole.

151. Social and Communal Life. Animals of the same
species often become associated even when there is no organic
connection between the individuals. The association may be
temporary or permanent. In its simplest form this is merely
a matter of gregariousness such as is seen in the schools of
fishes or flocks of birds, which are apparently brought together
at certain periods by a common instinct or by common needs.
A step more intimate is the banding together of predaceous
animals as wolves or vultures, or pelicans, for mutual help in
finding or capturing the prey. Corresponding to this, on the

9

114 ZOOLOGY.

part of their victims, we find the herding" of the bison, of deer,
and their allies for protection, whether by fighting together or
by the stationing of sentinels to give notice to the feeding herd
of the approach of danger. In still other forms, notably
among such insects as the bees and ants, there is a very intimate
and permanent union in social life. This is usually associated
with the instinct of home building, and thus a high degree of
division of labor with its great advantages becomes possible.
This is carried to such an extent that often polymorphic in-
dividuals result, much as in the organic colonies. In such cases
it is clear that the individual life comes to be bound up in the
success of the community. Such forms usually exert great
care for their young and develop a relatively high order of
intelligence. The principal social forms are the ants, of which
there are more than two thousand species ; some of the bees
and wasps; the termites, or so-called white-ants; beavers;
some monkeys and man.

152. Library Exercise. Make a report on the social life of the honey-
bee, including the following points : the home ; the kinds of individuals,
their origin, and their work in the community; their food and its prepara-
tion; mode of caring for the young; swarming and its significance. Make
a similar report concerning some species of ant. Find facts concerning
the following topics : " ants' cows " ; slave-making among the ants ; army
ants ; the agricultural ant.

153. Competition Among Animals of the Same Species
is not, for the most part, of a personal character except in the
case of the struggles of the males of polygamous animals.
The ordinary struggle for existence among them is merely
that of food-seeking, where all possess the same organs and
habits but in varying degrees of excellence. Those which have
the greater strength, hardiness, or intelligence are more likely
to get their portion of food at the expense of the weaker, and
thus to propagate their qualities. Sometimes however animals
live directly at the expense of their own species. Young
spiders before escaping from the cocoon in which they are
hatched devour each other, thus instituting an acute phase of
the struggle for existence in the place of the protection pre-

INDIVIDUAL DIFFERENTIATION AND ADAPTATION.

pared by parental care. Many fishes are known to devour
their own young. We have all had occasion to wonder what
becomes of the small frogs in a box containing large ones.
The struggle between the males for the possession of the
females has resulted in the development of many interesting
adaptations. The struggle may take the form of actual com-
bat in connection with which organs of offense and defense
are found. Such are the horns, tusks, spurs, manes, and even
the excessive size of the males as compared with the females.
Manifestly the same qualities which make a male a formidable
rival to another are likely to be of service to himself, his mates,
and his young, and thus to the species, in protecting them from
the attack of their enemies among other species. The com-
petition between males is not all of this stressful kind however.
It is believed by many naturalists that, in those instances where
simple mating rules, those males with the most striking colors,
pleasant voices, and winning ways displace their less favored
rivals and thus tend to accumulate by natural (sexual) selec-
tion the adaptations of this class.

154. The individuals of one species of animals may often
be practically indifferent to the presence of those of other
species. Their relation is simply that of competing for the
general food supply and thus assisting in the elimination of the
unfit in all species. They may graze in the same pasture, swim
in the same pool, or even be parasitic on the same host, and
have no other relation. From this as the simplest relationship
we may pass by gradual stages to the most intimate friendships
and the most bitter antagonism. Every species is indifferent
to some and hostile to other of the species which surround it ;
and man is no exception to the rule. It is a perversion of
manifest fact to pretend that all animals are of some use to
man.

155. We have seen that the individuals of a given species
are engaged in a struggle among themselves for the means of
subsistence, and that in certain cases they form communities
or colonies a kind of organic corporation in order to meet

n6

ZOOLOGY.

more successfully the demands made upon them by their en-
vironment. Similar partnerships may be formed by animals
of different species. The simplest of these associations are
known as commensalism or " mess-mat eism," in which the
degree of dependence and mutual advantage is perhaps not
very great. As instances may be cited the occupancy of the
same burrows by the prairie dog and a species of owl; the
attachment of barnacles to whales and sharks ; the hundreds of
species of other Insects which live in the nests of ants; the
lodging of fishes and other animals in the body-cavity of some
of the large tropical sea-anemones or among the tentacles of
some of the Hydrozoa. Each member of the association can
live without the other, but for some reason they often occur
together. The way in which species of rats and mice follow
man and occupy his habitations perhaps may be considered
under this head.

156. Symbiosis. Under this term are included even closer
relationships between members of different species, where there

FIG. 55-

FIG. 55. Hermit crab in the shell of a Gasteropod. After Morse.

Questions on the figure. What structural adaptations has the hermit-
crab to this mode of life? What conceivable gain has such a habit? What
animals are cited as symbiotic with the hermit-crab?

INDIVIDUAL DIFFERENTIATION AND ADAPTATION.

117

seems to be a distinct advantage accruing to both members of
the partnership sufficient to account for it. The relation
of the ants to the aphides or plant-lice which they capture may
be so described. The aphides, although captives, are nour-
ished, often at great expense of labor to the ant, on the food
which they most prefer, and in return the ants use the sweet
secretions of their bodies as food. Certain hermit-crabs,

FIG. 56.

FIG. 56. Argynnis cybele on thistle. Natural size. Photo by Folsom.

Questions on the figure. For what purpose does the butter-fly visit
the thistle? What special adaptations does the butter-fly possess for this
mode of life? What is the gain to the thistle from the visits?

whose habit it is to occupy gasteropod shells as a home into
which they insert the soft posterior part of the body, cultivate
friendly relations with a sea-anemone which becomes attached
to the shell, often with the active help of the crab. In this
case the anemone is supposed to conceal the hermit and to
help protect it by means of its nettling cells, and in return is

Il8 . ZOOLOGY.

carried about to fresh fields, and enjoys a portion of the food
broken up by the strong pincers of the crab. Observers claim
that the crab offers choice morsels of food to its companion.
When the crab by reason of its growth needs a new home it is
said to transplant the anemone thereto. These must be looked
upon as very remarkable adaptive instincts. Symbiosis is
probably more common between animals and plants than
among animals. The most interesting of these latter are seen
in the so-called " ant-loving " plants, in which the plant pro-
duces special homes or special foods for the ants, and the ants
in return protect the plant from the ravages of other leaf-
cutting ants or hurtful insects. Certain sea-anemones possess
unicellular algae imbedded in the cells of the entoderm. These
algae derive their nourishment from the wastes of the animal
tissues and supply oxygen and possibly other matter to the
cells in which they lie. The close relation between the struc-
ture and instincts of insects, on the one hand, and the form of
flowers, their products and needs, on the other, illustrates a
symbiotic adaptation which has long attracted students both
of botany and zoology. See Fig. 56.

157. Library Studies. Make a report concerning the various myrme-
cophilous plants. Accumulate all the supposed instances of symbiosis
which your library records. How do the lichens illustrate symbiosis ?

158. The Preying Habit. The effects of this habit are
stamped upon the structure and activities of both the pursuer
and the pursued. It is in this relation that nature is indeed
" red in tooth and claw." While in general the same organs
and habits which are of value in the capture of prey are useful
in the defense of the possessor, it is possible to find a series
of adaptations of an offensive character and others more
specially of defensive value. The curved claws and sharp
teeth, the stealthy approach, the sudden spring, and the great
agility of the one are met by the timidity, the keen senses,
the fleetness of the other. We can see that these defensive
adaptations must keep pace with the offensive else the prey
would be exterminated, which would entail no less surely the

INDIVIDUAL DIFFERENTIATION AND ADAPTATION.

destruction of their enemies than if these should lose their
power of capturing their prey.

159. Adaptations for Protection. In addition to the
alternatives of fighting or fleeing, the animals which are preyed
upon have very interesting and effective qualities that make
for safety. Many forms, as the Crustacea, have permanent

FIG. 57-

FIG. 57. Nestling Mourning Doves (Zenaidura macroura). From U. S. Dept. Agri-
culture Year-Book, 1900.

Question on the figure. Is there anything suggestive of protective
markings? What are the nesting habits of the dove? What character of
nest is constructed?

outer coverings; most mollusks have a box arrangement into
which they can retire when threatened by attack; others by
burrowing or otherwise come to occupy obscure corners in
nature where enemies find it difficult to follow. Forms as
widely different as the mole and the chamois find safety in
retirement. This hiding-theme may be wrought out in ways
almost equally effective by what is called protective resem-
blance. By this is meant that the animal becomes less easily

I 2O ZOOLOGY.

distinguished from its environment because of its color, or
form, or both. This resemblance may be to some particular
object, or merely a general harmony of color with the sur-
roundings. As illustrative of the latter head we may cite the
quail among the dead leaves and grasses, the sober-hued lizard
on the logs, the green caterpillars or tree-toads among the
leaves; the tawny color of desert animals, the white fur of

FIG. 58.

FIG. 58. A sea-horse, Phyllopteryx eques. From Eckstein.

Question on the figure. Compare this figure of sea-horse with figures
of other species and note the chief difference between them and the
typical fishes in external characteristics. What about the figure suggests
protective resemblance? At what point does the tail of the fish end?

arctic forms, the transparency of many marine animals. In-
deed the great majority of animals show some traces of re-
semblance to the surroundings, since it is alike advantageous
to the predaceous and to their prey. In some instances there is
the ability to change color with changing environment, as in
the tree-toads, the chameleon, and in some fishes. This is not
by the direct action of the light on the pigment cells but by
reflex action of the nervous system stimulated through the
eyes.

Many other animals become inconspicuous by reason of a
resemblance to special objects. It is among the insects that the

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 121

most numerous illustrations of this are found. The walking-
stick insect appears as dead twigs when not in motion. Many
butterflies resemble leaves, when at rest. A noted instance is
Kallima which is a large species, conspicuous when flying be-

FIG. 59-

FIG. 59. Walking-stick insect (Diapheromera veliei) on twig. Natural size. By

J. W. Folsom.

Questions on the figure. To what group of insects does this belong?
Do you see any reason to suppose that it illustrates protective resemblance?

cause of blue and orange patches on the upper surface of the
wings. The wings are folded when at rest and the lower sides
are colored and marked so like a dead leaf that the deception
is very complete. The larvae of some of the geometrid moths,
often called " measuring-worms," are remarkably like the
twigs on which they crawl, both in color and shape. This is
made more striking by the presence of roughnesses on the sur-
face which suggest buds, and by the possession of muscles
which enable them to support themselves rigidly outstretched
for hours by means of the posterior legs alone, so that the
axis of the body makes an angle with the branch.

Other instances of special devices whereby animals protect
themselves are found in the electric organs of some eels and
other fishes, in the poisonous fluids with or without special
stinging organs, as in ccelenterates, bees, some spiders, a few

122 ZOOLOGY.

fishes (spines), and some snakes; also in the repulsive odors
of the skunk and many caterpillars. Caterpillars oftentimes
have an acrid or otherwise unpleasant taste, but, unless this is
associated with a special odor or color by which its enemies
may recognize the fact, it is not likely to prove of any great
service to the animal possessing it since a single incision in
the soft body made by the bill of a bird is likely to cause death.
For similar reasons animals with stings are often highly
colored. The colors or other marks are, in these cases, in the
nature of warnings. The " monarch," one of our large con-
spicuous butterflies is an illustration of the association of color
and offensive taste ; the wasps and the coral-snake, of the asso-
ciation of color with the possession of stinging powers. Thus
owing to the power of association in the mind of the enemies,
the advantage comes to lie quite as much in the possession of
the special color or form as in the presence of the underlying
protective powers. These facts give rise to the remarkable
phenomena of mimicry. This term applies to those instances
where an edible or harmless animal, by reason of its similarity
to those which are disagreeable, partakes of their immunity
from attack. Mimicry must not be considered as in any way
a matter of choice with the animal but simply the result of
natural selection in preserving and allowing the propagation
of favorable variations. The viceroy butterfly, though edible,
seems to be protected by its striking likeness to the monarch.
The nearest relatives of the viceroy are quite differently
marked. Mimicry of bees and wasps is found among many
flies and some moths and beetles. Non-venomous snakes occa-
sionally have the marking and the motions of venomous.

160. Practical Exercise. Try to discover instances of general pro-
tective resemblance among the animals known to you. Analyze each case
and see just the nature and value of the projection. Treat similarly the
subject of special protective resemblance. Do you know any really harm-
less animals which assume apparently dangerous attitudes for protection?
Accumulate all the available references on mimicry. What range of color
have you seen illustrated among animals? In a single animal? Where,
on the earth, are the brightest-hued animals found? What are believed to

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 123

be the causes of colors among animals? What are the uses of colors?
What is albinism? Where have you seen instances of it? See Fig. 60.

FIG. 60.

FIG. 60. Albino Opossum (Didelphys virginiana). Photo by Folsom.

Questions on the figure. What is albinism? In what structures is it
manifest? Among what groups of animals can you find that it occurs?
To what is color in hair due? What natural conditions tend to produce
color in organisms? What is the chief value of color as an adaptation,
individual and social ?

161. Parasitism. Of a nature which combines the quali-
tes of commensals and of the preying animals is the association
known as parasitism. It is an association of individuals of
different species in which one member (the parasite) gets all
the benefits, and the other (the host) suffers the loss. It is
a case where one species preys on another, but in which it is
to the advantage of the parasite, especially if a permanent one,
as well as of the host that the life of the latter shall not be
suddenly destroyed. It will be readily seen that the parasite
increases the work to be done by the host, thus being a handi-
cap in the struggle for existence. This might easily bring
about the destruction of the species which serves as host were
it not for the fact that nearly or quite all animals support
various parasites. Parasites are of two classes, external, as
the fleas, lice, and the like ; or internal, as most of the parasitic
worms. The fleas are transient parasites, as are many other
insects which are free in the adult stage but lay their eggs in

I2 4

ZOOLOGY.

or on the body of the host where they undergo partial develop-
ment as parasites. In other instances the parasite must spend
its whole life in the body of one or more hosts. These are
called permanent parasites.

FIG. 61.

FIG. 61. Caterpillar of Platysamia cecropia parasitized. From Lugger.

Questions on the figure. Seek in your reference literature all figures
and references to caterpillars attacked by parasites. Why would cater-
pillars be rather favorable hosts for parasites? What are a few of the
parasitic enemies of caterpillars? What economic importance has this
phenomenon ?

In addition to the drain on the resources of the host, the
presence of the parasite may so irritate the tissues of the host
as to produce abnormal growth and disease therein. In many
of the transient parasites the life of the individual host is of
no consequence after the end of the period of parasitism and
hence the entire destruction of the host's body may occur just
as truly as in the ordinary preying species. Very profound
modifications occur in the structure of the parasite, which are
the outcome of, and in part an adaptation to, the special mode
of life. There is usually a degeneration of the organs of
digestion, of motion, and of sensation, since the parasite de-

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 125

pends on the host for the performance of these functions. The
explanation of this degeneration of useless or unused organs
is not quite certain. It is known that disuse causes structures
to deteriorate in the life of the individual, and some naturalists
claim that part of this loss is transmitted to the next genera-
tion. The claim is denied by many, who are disposed to con-
sider that it is merely a case of natural selection working for
simplification of organs and the economizing of materials.
The reproductive organs on the contrary become much more
complicated and the reproductive elements are produced in great
abundance. This is an adaptation to the difficulties involved
in finding the special host in which development may proceed.
This is more striking because many parasites require two dif-
ferent hosts in order to complete the life cycle, and great
mortality accompanies the passage from one host to another.
A good illustration of such parasites is the tape-worm which
infests the trout in Yellowstone Lake. The larvae enter the
tissues of the trout and by their ravages weaken and kill the
host. The dead fish are eaten by pelicans. The worms de-
velop to the adult, sexual condition in the digestive canal of
this second host and the eggs or young embryos escape into
the water with the excreta and from there are taken up by
other trout whose destruction is again wrought by the tissue-
infesting larvae. This passage from one host to another prob-
ably arose and is helped by the carnivorous habit among ani-
mals.

The parasites are almost exclusively invertebrates. The
worms and arthropods furnish the most numerous representa-
tives. The gregarines, among the Protozoa, are internal para-
sites, sometimes being parasitic within the cells. There are only
a few parasitic vertebrates, and these are transient. They be-
long to the lower fishes (lamprey, Fig. 62).

Parasitism is a very successful adaptation to a much limited
environment in which the organism has bartered its original
powers for a life of comparative ease. The only necessity
still resting upon it is in the matter of reproduction, and the

126

ZOOLOGY.
FIG. 62.

FIG. 62. Lake Lamprey (fetromyzon marinus unicolor) clinging to Sucker. (From
Bull. U. S. Fish Commission, by Surface.)

Questions on the figure. Does it seem that this is an instance of
parasitism or simple preying? What special organs has the lamprey adapt-
ing it to this habit? What references can you find to the breeding habits
of the lamprey?

success with which this needful function is accomplished shows
us that the parasite must be considered well adapted to its
conditions, notwithstanding its degeneracy. Its chief hazards
are met in the passage from host to host and these are over-
come by the carnivorous and omnivorous habits of hosts and
the extraordinary powers of multiplication on the part of the
parasites.

161*. Practical Exercises. Enumerate all the parasites, transient and
permanent, known to infest man, and find to what groups of animals they
belong. Report on the habits of the principal parasites on man : as tape-
worm, trichina, etc. What other hosts are demanded to complete the life
cycle? What are the principal sanitary conclusions to be reached? Ex-
amine the mouth-parts of the mosquito (see Fig. 63). To what kind of
feeding are they adapted?

162. Habits and Instincts in Relation to Adaptation.

In the study of adaptations there is constant danger lest we
come to consider that structures alone are adaptive. In reality,
adaptation in the manner of doing things is quite as important
as in the structure of the organs by which work is done.
When even the simplest organisms are acted on by an external

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 127

stimulus they respond to it in some way. This. response may
be either advantageous or disadvantageous to the organism.
If unfavorable, the result may be disastrous. If favorable
later repetitions of the stimulus are all the more likely to be
answered by the same kinds of response as in the first instance.

FIG. 63,

m. p.

FIG. 63. The head of female Mosquito (Cule.r). After Dimmock. a, antennae; c,
clypeus; h, hypopharynx; m, mandibles; ma., maxillas; tn.p., maxillary palpus; /,
labium; la., labrum (epi-pharynx).

Questions on the figure. In what way and for what purpose are the
mouth-parts of the mosquito used? What are the probable functions of
the antennae? Compare the antennae and the mouth-parts of a male and
female mosquito. See also Fig. 40. Mention some respects in which the
mosquito is adapted to its mode of life? What extent of horopter do
its eyes command? To what degree is the mosquito parasitic?

This individual acquirement of a special mode of responding
to stimuli is known as habit. Since responses in higher organ-
isms occur by means of the nervous system we rightly asso-
ciate habits with the nervous activities. In reality, however,
mere protoplasm may acquire these habitual modes of action
and one might say that all such adaptations are dependent on
the power of protoplasm to respond to external stimuli. By
reason of this power of adaptive responses, organisms may
become habituated or acclimatized to changes in their environ-
ment, their habits or responses changing according to the
necessities of the case. It is a matter of common observation
that animals can thus gradually be brought to the endurance
of conditions which would originally have killed them. Such
must have been true of the animals which have come to live

I 28 ZOOLOGY.

in the waters of hot springs. Such must have been the way
in which other animals were changed from the marine to the
fresh-water habit, since all fresh-water animals are believed to
have been derived from marine forms.

Similarly, in the history of any species those individuals
which respond in suitable or advantageous ways to the stimuli
brought to bear on them are selected from generation to gen-
eration in preference to those not so responding, and in the
course of time certain modes of action become characteristic
of the species, even without the necessity of individual ex-
perience. In other words the protoplasm has become so modi-
fied in a series of generations that responses of a definite kind
may be expected of it, which cannot be looked upon as in-
dividually acquired habits. These are instincts and embrace
many of the most interesting activities which have been men-
tioned as characteristic of animals. The instincts of feeding,
mating, and the like are examples. If instincts are in conflict,
the stronger prevails. In this possibility of situations arising
in which the instincts are in conflict, or are unequal to a correct
solution, lies the advantage of intelligence and choice as adap-
tations whereby correct responses may be made to external
conditions. Of the utmost importance in the development of
intelligence is the introduction of imitation, of training, of
experience, of memory, factors more or less represented in
the activities of all the higher animals. It is necessary to re-
member that what we call intelligence does not arise suddenly
in the animal kingdom and is not confined to the highest ani-
mals. Many of the acts usually spoken of as instinctive are
not purely so, but are the results, in part, of imitation, parental
or social training, and individual experience, and are therefore
to be classed as intelligent.

163. The Dispersal of Animals and the Formation of
Special Faunas. In section 1 36 we see that every point occu-
pied by the individuals of any species becomes, under natural
influences, a centre of distribution from which the species will

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 129

spread in all directions, unless kept back by adequate barriers.
Thus we should expect all animals to be found all over the
earth if all the conditions were equally suitable and all animals
were equally adaptable to varying conditions. This however
is not so. Species have unequal powers of adaptation to the
different conditions and thus it comes tq be that certain groups
of species adapted to some special environment will be found
together in certain regions, but will be absent from others.
The total animal life of any region is known as its fauna.

164. The Original Home of Animals, and the Sea-
faunas. There can be no reasonable doubt that animal life
began in the sea and close to its surface, and probably not close
to the shore. From this region the various nooks and crannies
of the earth have been occupied, until now it seems that there
is no place which does not have animals suited to its conditions.
The fauna of the surface of the mid-ocean is known as the
pelagic fauna. It is made up largely of Protozoa ; certain more
or less transparent types of invertebrates, as worms, jelly-
fishes, tunicates; many minute Crustacea and fishes. The
abyssal or deep-sea fauna contains representatives of all types
of animals from protozoa to fishes, notwithstanding the dark-
ness and the great pressure of the water. Many of the forms
are highly modified, differing markedly from the correspond-
ing species found in other life-regions. The littoral or shore
fauna is the most varied, abundant, and interesting of the
sea- faunas. Indeed there is no place on the earth where life
is more abundant. This is true because of the wonderful food
supply broken up by the waves, and the great variety of phys-
ical conditions at the meeting of land and water.

165. Library Exercises. What are the special conditions, of each of
the regions indicated in the preceding section, which are likely to be favor-
able or unfavorable to life? Illustrate more fully the typical forms char-
acterizing each region? Find instances of the special adaptations which
seem peculiarly advantageous to some of the animals frequenting each
region.

1 66. Fresh-water Faunas. From the littoral regions of
the sea, animals doubtless originally migrated into the brackish

130 ZOOLOGY.

water of the mouths of rivers. Thus certain types came to
inhabit the fresh waters of the streams, and as the result of
the adaptations thus made necessary new species arose dis-
tinctly different from their relatives which remained in the
sea. The most of the branches or phyla of the animal king-
dom have their fresh-water representatives, but very few
species of the sponges, the jelly-fish group, and none of the
star-fish group have left the salt water. Some species of
animals, as the salmon and eels, pass back and forth from fresh
to salt water in obedience to their spawning or other instincts,
but these are not very numerous.

167. From the fresh- water fauna or from the ocean shore
fauna have come those species which have acquired the power
of breathing by means of the air. These embrace some worms
and mollusks, the insects, and the vertebrates above the fishes.
This adaptation, which is one of the most important acquired
in the history of animal life on the earth, may have come about
by the gradual or periodic drying up of fresh water basins,
or by means of temporary excursions to the land, such as we
see some water forms capable of enduring today. Several
types of these terrestrial animals have achieved a more or less
complete mastery over the air (aerial fauna} by means of
flight. Chief among these are the insects, the first group to
accomplish the task; a group of reptiles in early geological
times; the birds; and a few mammals (as the bats). Animals
after passing from one region to another may in their descend-
ants reoccupy their old habitat. Thus the whales and seals are
air breathing mammals and are probably descended from land
forms, but have become aquatic. The same is true of some
reptiles. Many birds have lost their powers of flight and have
become purely terrestrial.

Other divisions of the continental and oceanic faunas into
geographical faunas are made, depending on the climatic con-
ditions and the geological history of the regions. The prin-
ciples governing this division are too complicated for our pres-
ent purposes.

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 13!

1 68. Summary.

1. It is necessary to consider the individual not merely as
a group of cells and tissues but as a unit acting and being acted
upon by all external forces and by other organisms.

2. Characteristics derived from the parents through the
union of the sexual cells, or by the non-sexual modes of repro-
duction, are described as hereditary. Other parental influences
have nothing to do with heredity. Heredity may preserve old
qualities or. give rise to new ones.

3. Individuals vary as the result (i) of internal conditions
and changes, the causes of which are obscure, and (2) of dif-
ferences in the environment. The environment may produce
very important changes during the single life of the individual.

4. The food supply of animals is limited, since all ultimately
depend on plants; any species multiplying at its average rate
of increase could in a short time, if unchecked, stock the earth
up to its limits of support ; that this does not occur is due to a
struggle for food among the excessive numbers which are
born, whereby only a small percentage of them reach maturity.
In the main, those survive which possess some qualities which
tend to fit them for the environment in which they find them-
selves. These are thus enabled to transmit their qualities to
their offspring, the fittest of which are again chosen. The
result is adaptation, and the process is known as natural
selection.

5. A similar result is effected by man in domestic animals
by artificially selecting individuals in accordance with the pos-
session of certain features. The resulting forms are frequently
very unsuited to the natural environment, and could not sur-
vive if left to themselves.

6. As the result of various causes animals become dispersed
from their point of origin, and in becoming adapted to the
different regions into which they go, or through variation
within a given region, give rise to new varieties. When, by
any means, these groups have become perfectly adapted to
their new special environment and permanently different from

132 ZOOLOGY.

their parent stock and from each other, without intermediate
individuals which manifestly connect the varieties, they are
recognized as new species. Through the influence of heredity
and by natural selection these differences may accumulate,
apparently to any amount.

7. The nutritive function relates particularly to the con-
tinued existence of the individual; the reproductive function
looks to the continuance of the species, and is a tax on the in-
dividual. Nature has specially favored those organisms in
which an increasing degree of energy is given to the produc-
tion of the young. As it is sometimes expressed, nature sacri-
fices the individual to the welfare of the species.

8. Animals become adapted to all the influences that tend to
make or mar their success in life. The more powerful the
influence the more certain the adaptation, because the destruc-
tion is the more certain in case of failure. The principal
classes of adaptations are, those relating to the using of the
favorable and resisting the unfavorable features of the inani-
mate environment; those assisting in the obtaining of food
whether vegetable or animal; those of mating and care of
young; those of offense and defense, in predaceous animals
and their prey. The relations and adaptations range all the
way from indifference to friendship, and from feeding at the
same talble on the one hand, to the utmost antagonism on the
other.

9. Perhaps the most important and the least understood of
the series of adaptations which animals acquire are those con-
nected with the nervous system and its functions : the habits,
instincts, and intelligence of animals. They are inseparable
from those already enumerated, and yet in fundamental im-
portance they form a group of their own. They seem pri-
marily to depend upon the irritability of protoplasm which
enables it not merely to respond but to become permanently
changed by that response a kind of organic memory. From
this fact acclimatization and adjustment become possible.

10. In being scattered from their starting place, animals

INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 133

with similar powers of response and adaptation come to be
located in the same kinds of conditions. This results in faunas
more or less characteristic of all the important kinds of en-
vironments : as marine, brackish water, fresh water, terrestrial,
aerial, cavern faunas, etc.

11. The origin of animal life was in the ocean, and from
these marine types it is believed that all other forms of animal
life have come, by gradual adaptation to their present mode
of life.

12. The various climatic zones of the earth and the principal
geographical regions are characterized by distinct forms of
life. For example, the lake life of Africa differs from that of
North America, and similarly for all the various types of
fauna. An analysis of such facts and an explanation of them
belongs to the geographical distribution of animals.

169. Topics for investigation, in field, laboratory and library:

1. What constitutes individuality in animals?

2. In what respects (enumerate) and to what degree have you ever
noticed variety in a given species? In the offspring of a pair of parents?

3. Have you ever observed any changes in structure in animals which
could reasonably be attributed to change in environment? Give evidence.

4. Does use or disuse produce changes in the organs of an individual?
Why? Give illustrations.

5. Enumerate some facts of your own observation which illustrate
heredity.

6. Cite observed instances of associations among animals of the same
species, and determine as well as you can from your observations what
ends are gained by the association.

7. Make an effort to classify a series of objects, noting carefully your
basis of classification; that is, the characters which you select in separat-
ing and grouping the individuals. The teacher can make this a most in-
structive exercise. A few objects of considerable diversity may be chosen,
as sand, pebbles, shells, crystals, a plant, an animal, and the student may
be required to examine each as fully as he can, write out the characters
which he discovers as belonging to each, being sure that he uses a simple
and observed feature in each statement. On the basis of these recorded
observations let him compare and group the objects. Or take a large
number of relatively similar individuals and, without stopping to write
their characters, let the student place or distribute them in groups near
or remote from each other in proportion to their unlikenesses, allowing
intermediate forms to stand between. Afterward he may be caused to
determine and justify his classification and to see whether other classifi-

134 ZOOLOGY.

cation could be made with a different basis. Gasteropod shells, illustrating
varieties of the same and different species; beetles, butterflies; grass-
hoppers; or even books of diverse shape, binding and contents may be
used.

8. Can you suggest any cause for the degeneracy of parasites?

9. Cite instances from your own observation in which animals use the
leap or spring in capturing prey or escaping enemies. Why is it a pecu-
liarly advantageous adaptation?

10. Cite instances of the food-storing instinct, with all observed details.
What is the most remarkable fact about them? How is it useful?

11. From reading and observation would you say that there is any
definite relation between the instinct for home-building and parental care?

12. Study sleep among animals. What is its relation to rest? Is it
found in the lower animals? To what is sleep an adaptation? When
does sleep commonly occur among animals? Why? Do plants show any
sleeping qualities?

13. What are the principal geographical faunas recognized by zoolo-
gists. Enumerate the more important means by which dispersal of ani-
mals from one region to another occurs. What are the chief barriers to
the dispersal of land animals? of aquatic animals?

14. Study the different authors to which you have access, as to the
significance of the terms species and variety (or sub-species).

15. What were the older views concerning the "fixity" or invariability
of species?

16. What are the different views of the " Origin of Species," as based
on the views of the meaning of species?

17. What is the essential difference between the theories of "natural
selection " and " definite variation " as explaining adaptation of organisms
to their environment?

18. What additional ideas are introduced by the "mutation" theory
of De Vries?

CHAPTER IX.
A GENERAL PREVIEW OF THE ANIMAL KINGDOM.

Before undertaking the study of the special groups into
which animals are arranged because of their apparent kinships,
it will be advantageous for the student to look briefly at the
whole field of animals, the " animal kingdom." See Fig. 64.

170. Mammals. Beginning with man himself it is easy to
see that there are numerous animals (as the apes and monkeys;
the various quadrupeds, as the horse, ox, dog, cat, bears and
squirrels ; the whales and seals ; and many others) which differ
much in general appearance from him but are like him in very
many remarkable particulars. For example, they all bring
forth their young alive and in a more mature condition than
is usual for other types of animals, the young being carried
in a special organ of the mother's body, often until develop-
ment is well advanced. After birth the mother produces milk
in special glands for the nourishment of the young to a still
more mature stage. This is seen in no other group of animals
beside the mammals. The skin produces hair or wool as a
covering for the body. Man differs from the other mammals
in certain particulars but not nearly so much as he and they
differ from other animals.

171. Birds. Another well-developed and numerous group
of animals is the class known as birds. There is scarcely an-
other class of animals so easy to distinguish at sight as this.
They equal or surpass the mammals in specialization, but are
very different from them. They are especially to be recognized
by the body-covering of feathers, the modification of the front
limbs into wings for purposes of flight, and the fact that the
jaws are sheathed in horny matter and, at least in present day
birds, do not possess teeth.

136

ZOOLOGY.

Reptiles

3000

'Amphibians

700

Arachnida

4.000

Molluskfi

Tunicata, etc. { 25000

400,

Crustacea

6000

rrtj , Trr I200

Ihread Pf^orms

Flat Worms
1700

Annelids /Molluscoidea

Coelenterates

Protozoa

4000

FIG. 64. Diagram showing the general relations of the chief divisions of the animal
kingdom. The number of species belonging to each is roughly approximate, only.

172. Reptiles. This is a class recognized by zoologists
which is not nearly so easy to define or to identify as either of
the preceding. This is partly because the animals composing
it differ more among themselves than in the other classes. It

GENERAL PREVIEW OF THE ANIMAL KINGDOM. 1,37

includes snakes, lizards, turtles, and crocodiles. The reptiles
have some features which indicate that they may be distantly
related to both birds and mammals, as well as to the next
class. This is an additional reason why the group of reptiles
is a difficult one to define. In general they may be recognized
by the fact that their bodies are covered by scales or plates
instead of hair or feathers. They always breathe oxygen from
the air, as do birds and mammals. They usually have only
three chambers to the heart whereas in the former groups
there are four. The blood is not constantly warm as in birds
and mammals. They lay eggs very much like those of birds.

173. Amphibians. In external appearance the members of
this class often look somewhat like reptiles, and they have
certain possessions in common with them, as the cold blood
and the 3-chambered heart. They are especially noteworthy
from the fact that they begin life breathing oxygen from the
water as fishes do, and later in life lose their gills, acquire
lungs, and get their oxygen from the air, as do the reptiles
and higher forms. Amphibians include the frogs, toads and
salamanders. This is not a very important group in nature,
but is intensely interesting to the student of zoology because
it seems to be a connecting link between the air-breathing and
the water-breathing forms.

174. Fishes. Fishes are characterized by the fact that
they breathe by means of gills throughout life. The body is
often scaly; the appendages are fin-like; the blood is cold, and
the heart has two chambers. They are beautifully adapted
to life in the water and are easily recognized.

175. Vertebrates and Invertebrates. All the animals of
which we have thus far spoken agree in certain particulars.
They all possess a dorsal rod of supporting matter (notochord;
see 341), which is often surrounded by cartilage or bone (the
vertebral column}. The nervous system in all of them is
dorsal to this rod and to the digestive tract, and is tubular in
character. The heart is ventral to the digestive tract and the

138 ZOOLOGY.

blood has red corpuscles. They are called Chordata or Verte-
brata. All other animals, with the exception of a few which
seem intermediate in some respects, are classed as Inverte-
brates, and agree in general in the following facts : there is
no notochord or vertebral column; the nervous system is
chiefly ventral to the digestive tract; the heart, when present
is dorsal; and the blood usually has only colorless cells. The
principal phyla of the Invertebrates follow.

176. Arthropoda. This is the most numerous phylum of
the animal kingdom. It embraces crayfish, lobsters, crabs
(Crustacea}, which for the most part have gills and live in
water; the Insects, as bees, flies, beetles, butterflies, etc., which
usually live in the air and get their oxygen from it ; the spiders,
whose habits and appearance are somewhat similar to those
of the insects. Arthropods are especially to be recognized by
the fact that their bodies are segmented, are bilaterally sym-
metrical, and have paired jointed appendages to many of the
segments. In addition to this there is a covering of resistant
substance (chitiri) developed by the skin. This serves for the
protection of the animal and for the attachment of the muscles
within.

177. Mollusca. This phylum of invertebrates includes the
snail, clam and oyster, the squid and devil-fish, and their kind.
They differ very much among themselves but agree in the lack
of segmentation of their bodies, in the absence of paired ap-
pendages, and in those types most commonly known to the
student, in the presence of a shell of one or two valves, which
is secreted by a fold of the skin called the mantle. While
many of the mollusks are lowly in organization and in intelli-
gence, one group of them that which includes the squid,
has the most highly developed brain found below the verte-
brates. It occupies among the invertebrates somewhat the
place which man has among the mammals.

178. Echinoderms. These are easily recognized by the
possession of five or more arms or rays in the adult stage.

GENERAL PREVIEW OF THE ANIMAL KINGDOM. 139

Usually a skeleton is developed in the skin. This is often cov-
ered with spines, and from this fact the phylum has its name.
They are marine and are poor movers, a few being fixed by
stalks to objects in the ocean. The starfish, sea-urchin and
sea-lilies are representatives.

179. Annulata (Segmented Worms). This phylum is
similar to the arthropods in that the body is bilaterally sym-
metrical, is segmented, and has paired appendages to many of
the segments. It differs from them in the fact that the append-
ages, when present, are not jointed but are merely setae or hairs
in sockets or on fleshy prominences. The segments are more
nearly homonomous than in typical Arthropods. The earth-
worm, many types of aquatic worms, and leeches are included
here.

1 80. Unsegmented Worms (embracing numerous ill-
assorted animals of doubtful relationship). Here may be in-
cluded a number of small groups many of which have long
been grouped with the Annulata and called " worms." They
are not sufficiently alike to be regarded as one distinct phylum.
The majority of them are bilaterally symmetrical, unsegmented
and without appendages. They differ from the Mollusks in
that they do not possess a mantle and do not secrete a shell.
Many of them are parasitic. Among these animals of doubtful
relationship may be included the " flat-worms," " round-
worms," the nemertea, rotifers, and others.

181. Coelomata and Coslenterata. All the animals thus
far considered possess during some stage of life a more or less
developed body cavity or ccelom (see 56) distinct from the
digestive tract. For this reason they are sometimes known
collectively as Ccelomata. All the remaining many-celled ani-
mals have a general cavity which serves both as a body cavity
and a digestive tract (gastro-vascular cavity}, or to speak
more exactly, there is no true body cavity. Of these the
phylum Ccelenterata are the chief illustration. Here belong
the jelly-fish, sea-anemone, corals. They are all aquatic and

140 ZOOLOGY.

are more or less tubular, sac-shaped animals often attached by
one end, with the mouth, which also functions as the anus, at
the other surrounded by clusters of tentacles. Many secrete
skeletons, and some form immense attached colonies.

182. Porifera. This group, to which belong the sponges,
is sometimes classed with the Coelenterata. While similar to
them in habit the sponges are much less highly organized and
unified. Instead of a single mouth opening into the digestive
tract, sponges have many openings or pores (whence the name
Porifera) which are the beginnings of tubes entering a central
cloaca or sewer. This is in reality not a true digestive tract.
It communicates with the exterior by one or more large pas-
sages. They are attached and usually form large colonies by
budding.

183. Protozoa. All the preceding phyla of animals con-
sist, in the adult stage, of many cells among which there is
more or less differentiation. In all of them the adult passes
through stages in which the cells are arranged in at least two
layers (ectoderm and entoderm^ see 53), from which the
tissue-masses arise. These animals are known as Metazoa.
In the remaining phylum the Protozoa the animals are
single cells, or at most loose aggregations of similar cells.
They are the lowest of animals and are for the most part in-
visible to the naked eye.

184. An Artificial Key to the Phyla of the Animal Kingdom.

Many-celled animals METAZOA.

With true coelom Coelomata.

Possessing notochord (and often vertebral column),

Phylum Chordata.
Possess functional gills.

Throughout life Class Fishes.

In embryonic life only (with a few exceptions),

Class Amphibia.
Do not possess functional gills.

Epidermal covering of scales Class Reptiles.

Epidermal covering of feathers Class Birds.

Epidermal covering of hair Class Mammals.

GENERAL PREVIEW OF THE ANIMAL KINGDOM. 141

Without notochord Invertebrata.

Bilaterally symmetrical (chiefly).
Body made up of segments.

Paired appendages jointed Phylum Arthropoda.

Paired appendages unjointed Phylum Annulata.

Body unsegmented; without paired appendages.
With mantle often secreting shell,

Phylum Mollusca.

No mantle Unsegmented Worms.

Radially symmetrical in adult Phylum Echinodermata.

Without true coelom.

With a single mouth, which also- functions as an anus : stinging

cells Phylum Caelenterata.

With numerous incurrent openings or pores, and only one or

few excurrent. No stinging cells Phylum Porifera.

Single-celled animals (chiefly) Phylum Protozoa.

CHAPTER X.

PHYLUM I. PROTOZOA.

LABORATORY EXERCISES.

Without compound microscopes this branch of animals can-
not be studied with profit in the laboratory. The Amoeba is
>ne of the most interesting of the Protozoa and serves well to
illustrate the simplest forms of animal life, but large specimens
in sufficient numbers for profitable study in an elementary class
are usually so difficult to secure at the right time that it be-
comes a question whether the teacher should be advised to
depend on them. My advice is, make every arrangement you
can to secure them, use them for demonstration or study
ivhenever they appear, but depend on Paramecium. Perhaps
the surest method for securing Amoeba is to chop up the soft
parts of three or four fresh-water mussels, placing the pieces,
together with the shells, in a large shallow basin. Allow a
gentle stream of water to drip into this. This keeps the water
slightly agitated, causes it to run over, and prevents an undue
accumulation of bacteria. The addition of a little of the sur-
face mud secured from the bottom of several streams or ponds
will make the success of the preparation all the surer.
Amcebas should appear at the surface of the mud, about the
shells, or at the margins of the vessel near the surface of the
water. Test all these places every day, and sooner or later the
Amoebas are practically sure to be found. Paramecia will be
likely to occur in the same preparation. Any abundant Proto-
zoan which may appear may be studied instead of Paramecium
or in addition to it, by means of the outline below. The mode
of securing the materials should be explained to the class to
make clearer the habits of these organisms.

185. Paramecium. This Protozoan may be obtained
readily by allowing fresh-water Algae, with hay or leaves, to

142

PROTOZOA. 143

decay in water. This infusion should be examined every day.
If the bacteria become too abundant some of the surface water
may be poured off and fresh water added. The paramecia,
which are just visible to the naked eye, appear as a whitish
cloud in the water or may accumulate as a film at the surface.
Often a sufficient number for study may be secured by scraping
with a scalpel the matter which accumulates on the sides of the
vessel just beneath the water surface, even when they are not
sufficiently numerous to cloud the infusion. The cover-glass
should be supported by sediment or by bits of cover-glass.
Make outline sketches of everything which can be thus shown.
I. With the low power of the microscope study the follow-
ing points:

1. Activities. Describe, and figure as well as possible, the
nature of all the movements of which the animal seems capable,
using arrows to indicate directions. Can you distinguish an
anterior from a posterior end? By what characteristics?

Do you find any reasons for believing that the Paramecia
are sensitive to external influences? What evidences? To
what sorts of influences do they respond? Do they avoid ob-
jects? Do they collide with each other in motion? Do they
tend to collect? Where? Are they as active at the end of the
hour as at the beginning?

Make a new preparation in which the Paramecia are uni-
formly distributed in a drop of water. Place a small grain of
salt at the edge of the drop. What is the result ? Watch the
individuals under the microscope as they come into the salt
solution. On a new preparation, try similarly a minute amount
of acetic acid (^ to y* per cent, solution) applied with a
capillary tube. Compare results. Try sugar, quinine.

Do you discover any instances of division or conjugation?
If so, describe.

2. General form of the body. How would you describe
its shape? To what degree is it capable of change? Is the body
symmetrical? Give evidences. Make diagrams showing your
idea of a cross-section through the middle; also of one, one-
third way from each end.

144 ZOOLOGY.

II. With the high power, study,

3. Cilia: where found? Are they uniform in length? How do they
act? What results do they produce? (Place a small amount of water
containing finely powdered indigo or carmine at edge of cover-glass. If
the movements are too rapid a little gelatine added to the water will be
of advantage.)

4. Find the mouth, with the oral groove leading to it. Position and
shape? How are food particles captured? Can you find them within the
body (food vacuoles')'? Do the food vacuoles move within the cell? If
so, trace their course? What finally becomes of them? Evidences?

5. Contractile vacuoles (clear spherical objects rhythmically disappear-
ing and reappearing). Number? Position? Rate of contraction? Do
they contract at the same time? What becomes of the clear material
during the contraction of the vacuole? Are they deep or superficial
structures? Your evidences? Does change of temperature cause any
change in their rate of contraction?

6. Distinguish between the inner mass of protoplasm (enclosure) and
an outer layer (ectosarc). What are the characteristics of each as re-
gards motion, clearness, firmness, etc.? Note the changes in these por-
tions on the addition of dilute acetic acid or iodine at the edge of the
cover-glass.

7. Discover if possible nuclear bodies. These are not usually recog-
nizable without careful staining. Place at the edge of the cover-glass, in
a fresh preparation of Paramecia, a 5-10% aqueous solution of methyl
green. Compare the result with a permanent mount stained by suitable
methods (see "Suggestions to Teachers").

186. Other Protozoa. If the class is supplied with microscopes, the
pupils should be allowed to examine stagnant water for as many types
of protozoa as may be found. Allow them to compare these, noting the
points of similarity and difference in general structure and activities.
Especially profitable protozoa for laboratory work are the green flagellate
infusorian, Euglena, which often tinges the water, or forms a green scum
over shallow pools of water; the colonial ciliate form, Vorticella, found
attached to submerged objects in ponds or pools of slowly moving streams
in which there is considerable decaying organic matter. The colonies are
easily visible to the naked eye. Stentor is a very large trumpet-shaped
infusorian which may be alternately attached and free-swimming. It lives
upon submerged sticks and leaves and may often be found attached to
the sides of vessels in which such matter has been placed. In all such
studies and identification of the protozoa the question of evidence of the
unicellular character of the organism should be kept before the student.

DESCRIPTIVE TEXT.

187. In this first and lowest group of animals, the individ-
uals of which consist of single cells or loosely associated simi-

PROTOZOA.

J 45

lar cells, we find something of the variety of shape which we
observed in the tissue cells of the higher animals (Chapter V).
The Protozoa are especially interesting to the biologist because
they represent the simplest forms of animal life now found
on the earth and because some of their representatives are very
like some of the simplest plants. Indeed some of them are
claimed by both the botanists and the zoologists. It also seems
probable that the first animal life to appear on the globe had
the general characteristics of some of the Protozoa. Whether
some type of protozoan is to be considered as the ancestor of
the higher many-celled animals or not, it is true that we find
illustrated here in the simplest possible way the beginning of
all those functions which are so completely distributed among
the special organs of the complex animals. The Paramecium
does in a simple yet satisfactory way all that any living animal
needs to do in order to live and perpetuate its species.

FIG. 65.

FIG. 65. Amoeba, ec., ectosarc; en., endosarc, containing food vacuoles (f) ; n,
nucleus; p, pseudopodium; p.v., pulsating vacuole.

Questions on the figure. Define the various terms used above in
describing the parts of the amoeba. What changes may the amoeba undergo
in its life history? Compare figures i and 6.

1 88. General Characters.

1. Mostly unicellular throughout life. May have one or
more nuclei (Figs. 66-69).

2. The protoplasm usually consists of a clearer outer por-
tion (ectosarc} and a more granular inside portion (endosarc)
(Fig. 66, ec, en).

11

146

ZOOLOGY.

3. There is usually what is known as a pulsating vacuole,
in which some of the more fluid cell-contents collects, to be
forced out of the vacuole again by the contraction of the denser

protoplasm (Fig. 66, pv}.

FIG. 66.

vn..

FIG. 66. Paramecium in optical section (semi-diagrammatic). A, anterior end; c,
cilia; e.c'., ectosarc; e.n., endosarc; f.v., food "vacuole"; g, gullet; N, meganucleus;
n, micronucleus; o, oral groove, leading to the mouth; p.v., pulsating vacuoles in dif-
ferent stages of contraction; tr., trichocysts; v, food vacuole in process of formation.

Questions on the figure. In what sense is the term " vacuole " descrip-
tive of the structures to which it is applied in Paramecium? Describe the
special adaptations of the anterior end. Judging from their distribution
have the cilia any other function than locomotion? In what way are the
food vacuoles formed? Why do some food vacuoles appear lighter than
others ?

4. Reproduction is effected chiefly by dividing into two or
more parts or cells, which occasionally remain associated. The
nucleus, when present, divides with the division of the cell
(Fig. 6).

189. Habitat. Protozoa in their active stages require
abundant moisture, hence they are found in water, fresh or
salt, and as parasites in the bodies of other animals. The
Sporozoa are parasitic. Some amoeboid Rhizopods infest the
digestive tract of man and other animals, producing irritation
and disease. The Infusoria occur in water in which there is
decaying organic matter and minute organisms of various
kinds. Volvox and Euglena, green forms often classed as
Protozoa, have the power which green plants possess of living
on the inorganic substances found in ordinary water.

PROTOZOA. 147

190. Organization. We cannot say that Protozoa have
organs in the sense in which we have defined that term hitherto,
yet they are certainly organized. The organization shows
itself in the nucleus, in the distinction of ectosarc and endosarc,
in the pulsating and food vacuoles, in temporary projections
of protoplasm called pseudopodia, in more permanent vibratile
projections of the ectosarc known as cilia or flagella, in the
mouth found in many forms, in cell-wall and secreted skele-
ton, in delicate contractile fibres in the ectosarc, and in stalks
for attachment to objects (see Figs. 66 and 68). By means
of these differentiations all the functions necessary to life are
performed. There are many colonial Protozoa. In such
(as Volvox} there may be some division of labor among the
cells, as between reproductive cells and body cells (Figs.
70,

191. Nutrition. The parasites absorb food, already di-
gested and fitted for absorption, directly from their hosts.
Most of the free forms take solid particles directly into the
endosarc through permanent or temporary openings in the
ectosarc. In some shelled forms, in which there is no mouth,
the food is digested outside the body proper (Fig. 72) by
the pseudopodia. These envelop the food and gradually trans-
fer it to the main body of protoplasm. In the other instances
the digestion takes place in the body of the protoplasm. The
ferments found in the protoplasm are doubtless responsible
for the digestive changes and act in much the same way as the
special ferments secreted from the cells of the digestive glands
in the higher animals. Circulation is effected by the general
protoplasmic motion. Respiration, whereby the protoplasm gets
rid of CO 2 and receives O, occurs through the cell surface
without special structures. All projections of the cell-body
assist in this exchange by increasing the area of the surface.
Excretion may take place from the surface of the cell, and it
seems probable that the contractile vacuole has an excretory
function.

ZOOLOGY.

192. Movement. The majority of Protozoa move freely
in their medium. In Amceba it is of a gliding character and
is effected by putting forth processes into which the protoplasm
streams. The process or pseudopodium thus enlarges at the
expense of the body of the cell and progress is had in the direc-
tion of the growing pseudopodium. The direction of motion
is changed by the breaking out of new processes in a new
direction. In those Protozoa which have a cell-wall special
devices become necessary to enable the animal to move. Most
of the free-swimming forms possess cilia or flagella, which
act as oars on the water and thus propel them. In Stentor,
Spirostomum, Vorticella, etc., there are clearly defined strands
of contractile material developed in the ectosarc by which the
shape of the animal may be strikingly changed. In the at-
tached forms these strands extend from the body proper into
the stalk. Vorticella (Fig. 68) by this device may change its
position with much suddenness. Attached forms are able to
break loose from their moorings and become free-swimming
for a time. Still other species are encased in shells and are
practically destitute of the power of independent motion.
Even the most active types may assume the non-motile or rest-
ing stage, by which they pass uninjured through such unfavor-
able conditions as drouth, cold, and the like.

193. Sensation. All the Protozoa show more or less sensi-
tiveness to external conditions. They may be caused to con-
tract and move by mechanical stimuli such as contact or
jarring, by chemically active substances in the water, by light,
by changes in temperature, and the like. Vorticella and
Spirostomum are exceedingly sensitive to contacts; Amoeba
avoids the light; many forms seem to find their food as the
result of the chemical differences in the water and may be
seen to swarm about suitable objects; the contractile vacuoles
of many forms contract more rapidly in warm than in cold
water; Paramecia tend to collect in groups at the edge of the
cover glass, around air-bubbles, about green filaments, or

PROTOZOA.

149

without any foreign matter. So far as we know, these simple
responses do not give evidence of special organs, but merely
represent a diffused protoplasmic irritability and power of
responding to stimuli ( 19, 20).

FIG. 67.

FIG. 67. Paramecium, i, transverse fission; 2-5, stages in conjugation. Lettering
as in Fig. 66. The meganucleus gradually disintegrates during the process and the
micronucleus by two successive divisions forms four micronuclei. Two of these dis-
integrate. One of the remaining micronuclei (n 3 ) in each animal passes into the
other Paramecium and unites with the stationary micronucleus (w 4 ), thus fertilizing it.
Later a new meganucleus is formed in each animal by the division of this body.

Questions on the figure. What structures divide in the fission of
Paramecium? Which do not? Which is permanently represented in the
cell during conjugation, the micro- or the mega-nucleus? Which seems to
correspond most nearly to the ordinary nucleus of higher forms? What
really transpires in the act of conjugating? Compare this with more
elaborate figures in reference texts.

194. Reproduction. In the Protozoa we discover methods
of reproduction which are to be looked upon as suggestions of
methods found in the Metazoa. Reproduction among the

150

ZOOLOGY.

FIG. 68.

FIG. 68. A, Vorticella, a stalked ciliate Infusorian: i, contracted; 2, extended. /,
food "vacuoles"; g, gullet; m, contractile fibre (muscular); n, nucleus; o, mouth, sur-
rounded by ciliated disc; p.v., pulsating vacuole; s, stalk. B, a colonial type similar
to Vorticella.

Questions on the figures. Compare the internal structure of Vorticella
with that of Paramecium (Fig. 66). What are the principal differences?
Likenesses? How is a colonial type (as B) formed? How are new colonies
started? In what way does the animal become extended after contraction?
Compare living animal.

FIG. 69. A, Euglena viridis, a flagellate Infusorian. i, typical swimming condition;
2, somewhat contracted; 3, spherical resting condition; 4, encysted stage in which fission
has taken place, c, cyst; /, flagellum; n, nucleus; o, mouth; p.v., pulsating vacuole;
sp, pigment spot.

B, Podophrya, a stalked Infusorian bearing tentacles (f). p, Infusorian captured
for food; s, stalk.

Questions on the figures. How does multiplication in Euglena differ
from that of Paramecium? What are the differences in the method of
feeding employed in Vorticella and in Podophrya? What is the structure
and function of the tentacles in the latter?

PROTOZOA. 151

Protozoa is, primarily, mere fission or division of the cell-sub-
stance. In some instances this division is little more than an
irregular breaking up or fragmentation of the protoplasm. In
others, one or more buds may arise from the parent cell. A
more typical method is by the equal division of the parent into
two new individuals. In still other instances, especially among
the Sporozoa, there is the formation of a cyst, within which
the protoplasm rearranges itself in numerous small bits which
finally break from the cyst as new individuals. In all such
cases the old nuclear material is distributed among the daugh-
ter individuals. There are indications that the process of
division carried on for a long time without cessation results in
a gradual loss of the vitality of the stock. There are two ways
in which this untoward result is overcome, so that a kind of
rejuvenation occurs. In the first place, a thick wall may be
formed and a period of rest ensue (encystment) . Or in the
second place, there may be a temporary (P arameciuvn) or
permanent (Volvox, Vorticella} union of two or more in-
dividuals. This is conjugation. The essential thing in con-
jugation seems to be the introduction of new nuclear matter
into the cell. The conjugation-cells' (gametes') may be alike
(Paramecium), or diverse (Vorticella or Volvox}. Parame-
cium may reproduce for many generations by division, and
then two individuals may conjugate, exchange certain nuclear
elements, and separate, beginning once more their process of
division. There is here no sign of sexual dimorphism. In the
colonial species however, as Vorticella and Volvox, there is
the union and permanent fusion of individuals (cells), dis-
tinctly different in form and size, to produce the new indi-
vidual. This is much like the dimorphism found in the sexual
cells in the Metazoa or many-celled animals, and illustrates
heterogamy (see 98). Consult Figs. 6, 67, 71.

195. History. The existence of the Protozoa was prac-
tically unknown until the compound microscope came into use.
A naturalist of Holland first discovered the Infusoria, and

I 5 2

ZOOLOGY.

thus opened up one of the most interesting departments of
zoology. It was not until the middle of the nineteenth century
that the simple, unicellular structure of the Protozoa was

FIG. 70.

FIG. 70. Eudorina. A colony of 16 flagellate cells imbedded in a gelatinous matrix.

FIG. 71.

FIG. 71. Eudorina. The development of reproductive bodies within the colony
from the ordinary vegetative cells (v). f, a mass of female cells; m, a mass of male
or motile cells; /', a single female cell surrounded by male cells (m') ; w, the
boundary of the original colony.

Questions -on .figures 70 and 71. What suggests that this is a colony
rather than an individual? What suggests the reverse? Compare accounts
in other texts to test your conclusions. What degree of differentiation is
shown among the cells?

PROTOZOA.

153

really, understood. Many of them can endure drying, be blown
about in the spore stage, and then take up active life again
on the return of water, so that thereupon, in a few hours, In-
fusoria may literally swarm where none seemed to be. This
is responsible for the long life of the old belief that they arose
by " spontaneous generation," that is, without parents. It is
only in recent years that this belief has been finally disproved..
It is known that they do not appear in water that has been
boiled and kept free from exposure to the air.

FIG. 72. A compound Foraminiferan Nodosarla. a, aperture of shell; /, food
particles captured by the strands of protoplasm outside the shell; n, nucleus; sh, shell.
1-4, the successive chambers of the shell; i, being the oldest.

Questions on the figure. Does this seem a colony or a single in-
dividual? Why? Why is digestion possible outside the capsule? Compare
this with figures of Protozoa in which there is no large aperture to the
shell.

196. Classification of Protozoa. The following are the principal
classes of protozoa.

Class I. Rhizopoda {root-footed'). Type: Amoeba. The Rhizopoda are

amoeboid in form with pseudopodia, which may be either blunt (Fig.

65) or slender (Fig. 72). The protoplasm may be naked (Amoeba)

'or may secrete a shell either calcareous (Foraminifera) or siliceous

(Radiolaria) . In the shelled forms the pseudopodia pass out through

ZOOLOGY.

openings in the skeleton (Fig. 73). Reproduction is usually by division,
or by the formation of many spores. Encystment frequently occurs.

Class II. Infusoria (in infusions}. Types: Paramecium, Stentor, Vor-
ticella. Predominantly active protozoa, usually without shell, but with
distinct cortical portion from which project permanent vibratile threads
of protoplasm (cilia, flagella, or tentacles), from the possession of which
the sub-classes are named. There is usually a permanent mouth. The
nucleus is always present and assumes a great variety of shapes. The
infusoria are typically free-swimming, but many are capable of attach-
ment by a contractile stalk, to foreign objects (V orticella) . Reproduc-
tion is normally by equal division, but budding and spore formation occur.
Conjugation is common, and may be either temporary or permanent.

FIG. 73-

FIG. 73. Actinomma, a radiolarian with a shell and no mouth. A, whole animal
with a portion of two spheres of shell removed. B, section, showing relation of proto-
plasm to the skeleton, c., central capsule; n, nucleus; p, protoplasm; o, openings
through which the pseudopodia extend. (From Parker and Haswell.)

Class III. Sporozoa (spore animals}. Protozoa predominantly passive
in habit, parasitic, with no pseudopodia, and no cilia in the adult. Re-
markable for encysted resting stages and spore formation. Conjugation
often precedes the formation of the cyst.

197. Place in Nature. Protozoa are an important element
in the food of many aquatic animals. Despite their minute
size, their immense numbers make them important. Together
with bacteria they serve to save for the organic world much
decaying material which no other animals could utilize. Rhi-
zopod shells dropping to the bottom of the ocean form the
" ooze," the chalk of later geological epochs. Other forms
of limestone also are produced by the accumulations of these

PROTOZOA. 155

calcareous shells. Similar masses of the siliceous shells occur
in various parts of the earth. Some of the Protozoa, especially
the parasitic Sporozoa produce diseases in man and other ani-
mals. Malaria and yellow fever in man are caused by Sporo-
zoa in the blood. In both these diseases, species of mosquitoes
are apparently the cause of the introduction of the spores into
the human system. Texas fever, one of the most dreaded of .
the diseases of cattle, is believed to be communicated through
the cattle tick, in which the sporozoan producing the disease
undergoes a portion of its life history.

Pieces of such protozoa as Stentor have been shown to be
able to regenerate a whole animal, provided a portion of both
nucleus and protoplasm are present, but not otherwise. This
shows that each is necessary to the activities of the animal.
Because they are lowly and simple animals, we must not con-
sider that they are either unimportant or unsuccessful in the
struggle for existence. Their wonderful reproductive power
insures that they hold their own whenever the conditions are
at all favorable for them. They occur in practically all the
waters of the earth, increasing or decreasing as their food
varies in abundance.

198. Supplementary Studies for the Library.

1. The reactions of Protozoa to light; to chemical substances; to heat;
etc.

2. Their power of resistance to heat; cold; drouth. The practical
results thereof.

3. The economic importance of Protozoa.

4. What is "plankton"? What is the importance of its study?

5. Conjugation in Protozoa. Compare methods of reproduction and
conjugation in the various groups. Follow the nuclear changes in con-
jugation of Paramecium.

6. Why should Volvox and Euglena be considered animals rather than
plants ?

7. Diseases in man or animals believed to be caused by the sporozoa.
The role of the mosquito in the life history of the sporozoa causing ma-
laria and yellow fever. The bearing of these facts upon infection and the
management of these diseases.

8. Forms of the Protozoa of different classes as shown by the illustra-
tions in the larger text-books.

9. The varying form of the nucleus in different species of Protozoa.

CHAPTER XL

PHYLUM II. PORIFERA.

LABORATORY EXERCISES.

199. Grantia. This is a marine sponge and in consequence
the majority of schools will be compelled to depend upon alco-
holic material. Grantia occurs along our New England coast,
and is found attached to piles or to stones a few feet below the
low-tide mark. If the school is near the coast the living sponge
should be studied in a basin of sea-water.

1. General Form. (Keep in a watch-glass, covered with
the preserving fluid.) Make careful outline sketches of every-
thing discovered.

Note, the basal or attached portion; the column; the free
end. How do the ends differ? Are there any openings?

Do you find any connection between individuals (budding) ?
Are these individuals of equal size?

2. Structure. Split the body longitudinally with a sharp
scalpel, and examine with hand lens or a low power of the
microscope.

Study, body wall; cloaca (internal cavity) ; the relation
of the cloaca to the osculum (the opening at the unattached
end).

By what is the osculum surrounded? Notice in the wall of
the cloaca the minute openings of the radiating tubes. Do
they communicate with the exterior? What are the functions
of the osculum and of the pores? Evidences?

3. Make thin cross sections with a razor, mount under cover-glass, and
examine further for points in 2. Notice the spicules. Is there any regu-
larity in their arrangement? What differences in shape and size have
you discovered in the spicules from different regions of the body?

4. Place a bit of the sponge in a small amount of a 5% solution of
caustic potash and boil. Examine under high power, and draw the dif-
ferently shaped spicules.

156

PORIFERA. 157

5. Place a bit of the sponge on slide and allow weak acetic or hydro-
chloric acid to pass under the cover. Note and interpret results.

200. Comparison Demonstrations.

1. Fresh-water Sponge. In portions of the country where the streams
are clear, swift, and with rocky bottoms, a fresh-water sponge may often
be found which will be valuable to compare with Grantia or substitute for
it. It grows attached to submerged objects and is commonly of a dirty
greenish color, though this may vary. This sponge is firm and gritty to
the touch, and may be either compact or branched. Use the general out-
line prepared for Grantia, noting the points of contrast. Is there any-
thing like the osculum? the cloaca? Gemmules or reproductive bodies
may occur imbedded in the flesh, especially at the base.

2. The Sponge of Commerce. This is merely the skeleton of a sponge
from which all the cellular part has been removed. Select a small rounded
specimen. Do you find any signs of the attached end? of an osculum?
Split the sponge with scissors, beginning with an osculum. Are there any
canals as in Grantia ? If so, what is their arrangement? Examine a small
portion of the skeleton under the microscope. Test as before (for calcic
carbonate) with dilute acid. Is the skeleton elastic? Why?

DESCRIPTIVE TEXT.

201. The Protozoa are unicellular animals, or at most,
masses of similar cells in a more or less globular form. This
condition is comparable to the morula stage of the embryos of
higher animals (see 52). In all the other groups (Metazoa)
the cells at some stage in development are in at least two
layers, an inner, and an outer or superficial layer, a structural
condition which we have seen at its simplest in the gastrula
(see 53). The exact position of the Porifera in the animal
series has long been a matter of debate, but the great majority
of zoologists agree that they stand below all the other Meta-
zoa, presenting transitional features between the Protozoa
and Metazoa. For this reason they are especially interesting.
Some authors include them with the next phylum the Coelen-
terata. They possess two cell-layers, but the division of labor
among the cells is not so decided as in the Crelenterata, and
the individual cells are very much more independent of each
other in consequence.

202. General Characters.

i. Porifera possess a system of internal chambers through
which the water flows. The water enters by means of many

ZOOLOGY.

minute pores at the surface, passes along radiating tubes (in-
current channels) to the central cavity (cloaca) and escapes
through one or more larger openings (oscula) at the unat-
tached end. There is no true ccelom (see 56).

2. Parts are arranged about the central cavity but not usu-
ally in a symmetrical fashion.

3. There are two distinct layers, ectoderm and entoderm.
These are separated by a gelatinous mass in which are in-
cluded cells of different kinds (mesenchyma or mesoderm)

FIG. 74-

FIG. 74. Leucandra, a simple type of sponge. (From Delage and Herouard; " Traite
de Zoologie Concrete.")

Questions on the figure. What is the position of the osculum ? Which
is the attached end? How many individuals are represented in the cut?

FIG. 75. Diagrams to illustrate the development of one of the simpler types of
sponge: i, the egg; 2, section of 16- to 32-celled stage; 3, section of later stage, a
ciliated larva (blastula) ; 4, gastrula; 5, section through older larva which has become
attached by the end containing the blastopore. New openings break through by the
coalescence and perforation of the ectoderm and entoderm, and a form results such
as is figured in Fig. 76. a, archenteron; bl., blastopore; ec., ectoderm; en., entoderm;
mes., mesenchyma; s, segmentation cavity.

Questions on the figures. What terms would be applied to the cleav-
age and gastrulation in this sponge? What is suggested as to the mode of
forming mesoderm? The attachment of the sponge by the blastopore end
of the larva necessitates what later development? See Fig. 76.

PORIFERA.

not in a true layer. In the cells of the mesenchyma spicules
are produced, forming the supporting skeleton (Fig. 77, C).

4. Non-sexual reproduction is prevalent, but dimorphic
sexual cells are also formed in the mesenchyma. The sexually
produced larva is free-swimming; the adult is attached.

5. Mostly marine; wholly aquatic.

FIG. 76. Diagram of simple type of sponge, more mature than in Fig. 75. c,' cloaca;
ch, chambers, lined with flagellate entoderm; e.p., external pores; i.p,, internal pores;
mes., mesenchyma; o, osculum; r.c., radiating canals. Other letters as in Fig. 75. In
the adult sponge the canals and flagellate chambers become much more complex than
figured here.

Questions on the figure. What portions of the animal are lined with
ectoderm ? With entoderm ? What two main types of entoderm are figured ?
What is the actual nature of the mesoderm in sponges? Is there a coelom
(a cavity bounded by mesoderm) ? What mechanical advantage do you
see in the fact that the water currents enter by way of the radial canals
and find their exit through the osculum, rather than the opposite direction?
Compare with figure 77.

203. General Form. The simpler sponges are cylindrical
or vase-shaped sacs with an opening (the osculum) at the un-
attached end. From the central cavity (cloaca) of the sac
numerous radial passages pierce the walls (Fig. 76), and

i6o

ZOOLOGY.

terminate directly or indirectly in pores at the surface (whence
the name Porifera). In the more complicated sponges there
is such power of budding and lateral growth that there is
formed a dense tuft of sponge made up of many individuals
in organic connection with each other. In such sponges the
simplicity of the internal structure is lost, and the cloaca may
branch, opening to the exterior by a number of oscula. The
radial passages which penetrate the wall become much
branched and enlarged in special regions until the mesen-
chyma becomes honey-combed with the passages and cham-
bers. No animals are more profoundly influenced by their
environment, in the matter of the special form which the
individual assumes, than the sponges. Individuals which de-
velop in active currents differ much in bodily shape from
members of the same species which grow in sheltered places.
In all instances the form assumed appears to be correlated to
the external conditions.

204. The Structure of the Body. In the typical condition
the body of a sponge consists of ectoderm and entoderm,
with a gelatinous mass between them in which are imbedded
cells of various kinds and spicules of hard material forming
a skeleton (Fig. 77, C). The ectoderm is usually of flattened
cells and covers the exterior. It lines the pores and the outer
ends of the passages by which the water passes to the interior.
The entoderm lines the cloaca and the radial tubes, and is
especially well developed in the pocket-like enlargements of
these tubes, when they occur (Figs. 76, ch; 77, en 2 }. In these
passages the entoderm is more columnar in shape and is sup-
plied with flagella, by the action of which currents of water are
kept flowing inward to the central cavity. The middle mass or
mesenchyma, which lies between these two layers and makes
up the principal thickness of the body, consists of numerous
cells of various kinds in a gelatinous intercellular substance
(Fig. 77, mes). Some of these cells are amoeboid or migra-
tory, others resemble the cells of connective tissue, others se-

PORIFERA.

T6l

crete the spicules which form the skeleton, and still others are
reproductive. The spicules of which the skeleton is made are
different in different classes of sponges. They may be cal-
careous, siliceous, or horny. The sponge of commerce illus-
trates the last class. The spicnles may be isolated and inde-
pendent, or become fused into a continuous skeleton. It is
the skeletal part which prevents the otherwise soft animal
from becoming collapsed into a shapeless mass, and thus en-
ables the cavities, by means of which nutrition is effected, to
be kept open. It is the variety in the skeletons, too, which
gives the diversity of form seen in the individuals of different
species.

205. Nutrition. The food of sponges is essentially similar
to that of the single-celled Protozoa. It is carried in by the
water currents, which enter the pores, pass along the canals
lined with flagellate entoderm into the cloaca, and from there
reach the exterior by way of the osculum. The food particles
are taken up principally by the entoderm cells lining the radial
chambers and by the amoeboid cells which belong to the mesen-
chyma. In these cells digestion takes place as in Amoeba. The
indigestible parts of the food are returned to the current and
are eliminated through the osculum. There is no circulation.
The digested food diffuses from cell to cell or is carried by the
amoeboid cells. Respiration occurs through all the cells which
are in contact with the water.

206. Sensation and Motion. Sponges are fixed and vege-
tative in their adult life, and show very little of the more
active functions. In addition to the ciliated and amoeboid cells
already described, the pores may be closed in response to
stimulus. Both nervous and muscular elements have been
described as occurring in these regions, but there is some ques-
tion as to the degree of their structural differentiation.

207. Reproduction by outgrowth or budding is common.
In this way large colonies arise from a single individual. New
colonies may arise, especially in the fresh-water sponges, by

l62

ZOOLOGY.

the separation of gemmules or groups of cells produced asex-
ually within the mesenchyma. These, after a period of rest
escape and produce new individuals. Sexual reproduction also

FIG. 78.

FIG. 77. Diagrams showing the arrangement of the radiating canals in two types of
sponges: A, Ascon type; B, Sycon type; C, a portion Or) of the latter, more highly
magnified, showing character of the three layers, ec., ectoderm; ent, entoderm (flat-
tened layer); en 2 , flagellate entoderm; e.p., external pores; i.e., flagellate chambers
of the radiating canals; i.p., internal pores; mes., mesenchyma; r.c., radiating canals;
D, two flagellate cells more highly magnified. After Korschelt and Heider.

Questions on the figures. Trace the relation of ectoderm to the
entoderm in these two types? Compare these with illustrations in refer-
ence texts. Is there any way of accounting for this disproportionate
growth of the entoderm ? What are the apparent functions of the flagellate,
collared epithelium? What structures are to be found in the mesenchyma
in sponges?

FIG. 78. Axinella polypoides, showing numerous oscula. After Schmidt.

Questions on the figure. What are the principal external differences
between Axinella and Leucandra (Fig. 74) ? How many individuals are
represented here? What are the grounds for your answer? Compare this
with the skeleton of the sponge of commerce.

PORIFERA. 163

occurs in all sponges. The ova and sperm are developed in
the mesenchymatous layer. The male and female cells orig-
inate from the same individual (hermaphroditism} . Usually
however the sexes mature at different times.

208. Development. Fertilization of the ovum and early
cleavage take place in the mesenchyma near the incurrent
canals, by means of which the spermatozoa find entrance.
Cleavage is total and for the most part equal (see 51), pro-
ducing an oval blastula which swims freely by means of cilia
or flagella. While there are some peculiar features about the
gastrulation, a gastrula or two-layered embryo is ultimately
formed. At this stage the embryo settles to the bottom and
becomes attached by the end containing the blastopore, which
thus becomes obliterated (Fig. 75, bl). An excurrent pore
breaks through at the opposite end, and the numerous incur-
rent pores are formed at the sides. The mesenchyma seems
to be formed by cells which migrate from the other layers
into the segmentation cavity, thus filling it. The entoderm
outpockets into the mesenchyma, establishing connection with
the ingrowing ectoderm, thus forming the incurrent canals
(see Fig. 76). In most species the process is more complex
than that described here.

209. Classification.

The divisions of the group Porifera are made on the basis of the dif-
ferences in the skeleton. Two principal classes may be recognized, as
follows :

I. Calcarea. Sponges in which the skeleton is composed of calcareous
spicules. Laboratory type, Grantia.

II. Non-Calcarea. Sponges with glassy (siliceous) spicules, or with
horny (spongin) fibres, or with merely a gelatinous mesenchyma. Labo-
ratory types : the fresh-water sponge ; the commercial sponge.

210. Sponges are chiefly marine animals, and flourish in
all the seas and at any depth. The larger horny sponges of
which the bath sponge is the skeleton are found in the warmer
seas, and in relatively shallow water. By reason of their
budding and branching, the sponges form immense colonies
or beds, and many other forms of life associate with them in

164 ZOOLOGY.

varying degrees in intimacy. Fossil sponges, apparently of
the same general characteristics as those now living, are found
in very early geological strata.

211. Supplementary Library Studies.

1. Economic value of sponges. Sponge fisheries. The mode of pre-
paring sponges for market.

2. What arguments may be advanced for considering the sponges as
colonial Protozoa? What is the conclusive argument for regarding them
as Metazoa?

3. By comparing the figures of sponges found in your reference books,
note the different degrees of development of the passages lined with ento-
derm and ectoderm in the walls of various species.

4. In what special ways do sponges become adapted to the conditions
in which they are situated? Effect of rapid currents on them? Of quiet
water? Of muddy water?

CHAPTER XII.

PHYLUM III. CCELENTERATA. (HYDROIDS, CORALS, JELLY-
FISHES, ETC.)

LABORATORY EXERCISES.

212. Hydra. Hydras are small tubular animals found in
permanent fresh-water pools, attached to submerged leaves,
twigs, algae, etc. They are somewhat difficult to recognize
when disturbed because they contract into small rounded
masses, close against the supporting object. Promising ma-
terials should be collected from several ponds, and placed in
shallow vessels (a white-ware dish is good), and in a short
time the hydras will become extended. The green hydra
(H. viridis} is perhaps more common and hardier, but is not
so satisfactory for general laboratory work as the brown
(H. fusca), because it is less transparent.

i. Study the living animal in a glass jar (tumbler).

Is it free or attached? What happens if it is freed from its
attachment? Is it lighter or heavier than the water? Evi-
dences. Can it move from one portion of the vessel to an-
other? If so, does it become detached? Watch same individ-
uals from day to day. What is its position in the water?
If the vessel containing hydras be placed near the window, at
which side of the vessel do the animals become collected?
When the animals are stretched out at their greatest length,
touch lightly the tip of one of the tentacles. Touch the body.
Repeat the experiment until you are sure of your results.
Note and explain as well as you can the results. Of what
degree of contraction is the animal capable? Do you notice
any contractions or motions of parts, when the hydra is un-
disturbed? What seems to be the purpose of the motions?
Evidences? Bring a piece of meat the size of a pin-head or a

Daphnia or Cyclops in contact with the tip of a tentacle and

165

I 66 ZOOLOGY.

note the results. How do the other tentacles behave? Place
a food-particle directly at the base of the tentacles. How is
it swallowed? How long does it take? What becomes of it?
How long does it remain in the body? Classify the results
which you have attained, under the following heads : motion
and locomotion, nutrition, sensation. Devise still other ex-
periments to test special points which you desire to know.

2. General Structure. Transfer a living animal to the
slide, covering it with a drop or two of water. Observe with
a low power without cover-glass. Draw carefully in outline
everything studied.

Note body regions:

Foot (attached end).

Column.

Tentacles, position, number (examine several specimens).

Hypostome, surrounded by the tentacles.

Mouth.

To what extent do these regions vary in their dimensions
during the different stages of contraction of the hydra?
Would you say there is any distinct symmetry? Which is the
main axis? Is there any indication of an internal (gastro-
vascular} cavity? What is its extent? Are the tentacles solid
structures? Evidences? Are there any buds in your speci-
men? Relation to the parent? To what extent do different
parts of the body do different work?

3. Microscopic Structure. Cover with a cover-glass supported by
objects as thick as the animal. Study with a higher power. Verify the
points studied above. Follow the gastro-vascular cavity more fully. Is
there an aboral opening?

Body wall.

Ectoderm, or outer layer of cells.
Entoderm, or inner layer of cells.

Determine the extent of each layer. Are they continued into the ten-
tacles? What differences do you find in the thickness of the layers and
in the shape and character of the cells of each layer in the various parts
of the body? Is there anything between the ectoderm and entoderm?

In the ectoderm, especially in the knobs on the tentacles, find highly
refractive oval bodies, the nettle capsules. Irrigate with a drop of acetic
acid, and watch the tentacle all the while. What changes have occurred

CCELENTERATA. 1 67

in the nettle cells? [A whole animal stained and mounted may be studied
profitably in comparison with the preceding.]

4. Histology from Sections. If the teacher is not equipped for im-
bedding and sectioning objects, and desires to carry this work further,
stained and mounted sections of Hydras and most of the other prepared
sections suggested in this book can be secured for a reasonable sum by
applying to any of the large laboratories. By comparison of longitudinal
and transverse sections verify your observations concerning the extent of
ectoderm and entoderm. What occurs between the layers?' Study the
shape and arrangement of the cells in both layers. Compare as to size.
What is the relation of the nettle cells to the other ectodermal cells?

5. Histology from Maceration Preparations. Place a specimen in a
watch glass, and draw away some of the water with a pipette. When the
Hydra is well extended, pour over it an aqueous solution of hot corrosive
sublimate. Rinse and place in Muller's fluid or 15% alcohol for 24 hours.
Take a portion of the body and place on a slide in a drop of glycerine
and water. Cover, and tap the cover-glass very gently with a needle.
The cells thus become separated, and their shape may more readily be
seen. Instructions for staining may be found in texts on histology.

Study the nettle cells, the ectodermal cells, the entoderm, and the
gland cells of the foot and gullet.

213. For comparison with Hydra the teacher should secure some alco-
holic material of some of the marine hydroids, as Pennaria, Obelia or
Campanularia. A few slides should be secured bearing whole mounts and
sections properly stained.

The following points should be studied briefly: Relation between indi-
viduals in the colonies, branching. What classes of individuals are dis-
coverable, i. c., how do the different branches end? Is there any cover-
ing to the softer portions? Tentacles; are they present? If so, what is
their arrangement? Hypostome? Mouth? Is there a gastro-vascular
cavity? Ectoderm? Entoderm? Call attention to polymorphism among
the polyps or zooids.

214. Metridium (Sea-anemone). If lack of appropriations will not
allow the purchase of sufficient material for class work, the teacher should
have at least a few well hardened and preserved specimens of sea-ane-
mone. From these should be made a series of cross-sections from various
parts of the body, with a thickness of one-eighth to one-fourth inch.
These sections may be fastened to cards or to plates of mica by thread
or fine wire and kept in preserving fluid. One specimen should be split
lengthwise, and one left whole. Four . or five specimens could thus be
used from year to year until more abundant supplies are obtained.

The following studies should be made. . Make drawings to illustrate
all points made out.
i. General Form.

Base, or aboral disc (the end attached during life).
Column.

I 68 ZOOLOGY.

Oral disc: zone of tentacles; intermediate zone; lip-zone; mouth;
siphonoglyphs (grooves in the angles of the mouth), number?

2. Transverse Sections.
Body wall.

(Esophagus; does it appear in all the sections? Siphonoglyphs?

Mesenteries. How is the oesophagus held in position? What dif-
ferences do you find in the mesenteries? They are described as
complete (or primary), and incomplete (or secondary, tertiary,
etc.).

Show by a diagram the number and arrangement of them, especially
of the primary. Are they in pairs? Notice the inter-mesenteric
chambers. Can you find the muscular thickenings in the cut mes-
enteries? Sketch their position. Compare with conditions figured
in various text-books.

3. Longitudinal Section.

Complete your study of the structures mentioned above.
Compare the complete and incomplete mesenteries.
Identify :

Mesenteric filaments (on free edge of mesenteries).
Genital glands (developed in the substance of the mesentery near

the edge).

Ostia, or ring canal ; openings through the mesenteries by means
of which the mesenterial chambers communicate with one an-
other.
Are the tentacles solid or hollow?

4. General Considerations.

Make diagrams in longitudinal and transverse view to show the dis-
tribution and connection of the cavities of the body. Is the mouth the
only opening into the cavity? Describe the symmetry of the anemone.
Is it radial or bilateral? Give reasons for your answer.

215. Oculina (or other branching coral). Study the branches and
note the position of the polyps. Is the arrangement orderly? If so,
describe.

Note with a hand lens the arrangement of the septa, which grow be-
tween the fleshy mesenteries of the coral. Compare their arrangement
with that of the mesenteries of anemone.

DESCRIPTIVE TEXT.

216. Some authors place the sponges and the ccelenterates
in the same group on account of the typical barrel shape, the
absence of a true ccelom or body cavity, the somewhat similar
character and origin of the middle mass (mesenchyma), and
the agreement of the principal axis of the adult with that of
the gastrula. In the ccelenterates however there are no lateral

CCELENTERATA.

169

pores. The principal opening serves as a real mouth as well
as vent for the voiding of undigested matter, whereas in
sponges it is not a mouth in any sense. In general the indi-
vidual even in the colonial forms of ccelenterates is more dis-
tinctly an individual than in the sponges. The division of
labor among the parts and the interdependence of parts is
rather greater than among the sponges.

FIG. 79.

H.

"v

FIG. 79. A, Longitudinal section through the body of Hydra (diagrammatic). B,
small portion of the wall more highly magnified, b, bud; ect., ectoderm; ent., ento-
derm; f, foot; //., flagellum; g.v., gastro-vascular cavity; m., mouth; mes., mesenchyma
(noncellular) ; m.f., muscular processes of the ectodermal cells; n, nettling cells; n',
same, exploded; nu., nucleus; t, tentacle; v, vacuole.

Questions on the figures. How many cellular layers are to be dis-
tinguished in Hydra? What differentiations are represented in the
ectoderm in different regions? In the entoderm? What is the relation of
the bud to the adult? Why is the cavity called a gastro-vascular cavity?
How is contraction effected in Hydra?

17 ZOOLOGY.

217. General Characters.

1. A single system of internal chambers (gastro-vascular
cavity} in which digestion and circulation both occur. No
ccelom.

2. Parts radially arranged about an oral-aboral axis. Ten-
tacles usually occur at the oral pole (Figs. 80, 83).

3. A supporting layer or mass (mesenchyma) between
ectoderm and entoderm, sometimes without cells. More often
cells of various kinds occur, which have migrated from the
other layers.

4. Nettle cells are found in practically the whole group
(Fig. 81).

5. Nerve cells (sensory) and muscle cells both occur.

6. Reproduction by non-sexual methods is prevalent. This
often alternates regularly with the sexual. The individuals of
the two generations may be very different in appearance and
habits.

7. Wholly aquatic; chiefly marine.

218. General Survey. The group of Qelenterata em-
braces animals very diverse in general appearance, which may
nevertheless be reduced to two types. The first and most
primitive is the tubular hydroid type. This is sessile and is
essentially a gastrula, at the unattached end of which occurs
the mouth, usually surrounded by tentacles. The cavity of
the tentacles is continuous with the gastro-vascular cavity
(Fig. 79). Of this type we may distinguish two conditions:
(i) in which the individuals (polyps) occur singly (Hydra},
or if in colonies, the various individuals have the same form
(as the corals} ; (2) colonial forms in which the individuals
making up the colony are very different (as the Siphonophora} ,
embracing open-mouthed nutritive individuals, mouthless re-
productive polyps, protective polyps abundantly supplied with
nettle-cells, bladder-like supporting polyps, etc. (Figs. 84, 85).
The extreme conditions of (i) and (2) are connected by
types possessing intermediate degrees of polymorphism.
Though the individual polyps are attached, the whole colony

CCELENTERATA.

171

--WUS.

FIG. 80. Sections of types of Ccelenterates (diagrammatic) : i (longitudinal) and 2
(transverse) of a tubular hydroid; 3, Sea Anemone (longitudinal); 4, same (transverse,
at the level of the upper dotted line) ; 5, same (transverse, at the level of the lower
dotted line); 6, longitudinal or vertical section of a Medusa; 7, transverse section
of same at the level of ih$ dotted line. The continuous line is ectoderm, the broken
line, entoderm, and the stippled portion, mesenchyma. c.c., circular canal; g, gullet;
g.i'., gastrovascular cavity; m, mouth; ma., manubrium; mes., mesentery; mes. 1 , direc-
tive mesentery; o, ostium; r.c., radial canal; *, tentacle; v, velum.

Questions on the figures. By a careful comparison of the diagrams
what points of similarity do you find in these three types? What are the
principal points of difference? Examine similar diagrams in other texts.
Why is Coalenteraie an appropriate name for all?

may float freely. The second type is the active jelly-fish, or
medusoid (bell) type. The medusae, though varying greatly
as to details agree in having a shape comparable to that of
an umbrella or a bell. The convex surface is normally the
upper surface. At the margin of the umbrella are tentacles
often very numerous, and frequently much elongated. In the
middle of the concave surface is a projection, at the lower end
of which is the mouth-opening. The gullet leads from the
mouth into a cavity in the central portion of the body of the
bell (gastro-vascular cavity}. From the central cavity

I7 2 ZOOLOGY.

radiating passages run through the substance of the bell to
the margin where they may communicate with a circular canal
which passes around the bell near the bases of the tentacles.
This whole internal cavity is lined with entoderm, and there-
fore no portion of it represents a ccelom, but is merely a much-
modified digestive tract (Fig. 80, (5).'

The bell is comparable to an inverted polyp in which the
main axis has become much shortened, accompanied by a
thickening of the body in the direction of the other axes. 1
The gastro-vascular cavity is further modified by the increase
of the mesenchyma of the aboral disc and by a union of the
oral and aboral walls of the cavity in certain regions. The
large chambers between the mesenteries in such forms as the
sea-anemone thus become limited to small radial canals. Fre-
quently both of these types are found in the life history of the
individuals of a single species. The tubular colonial polyp
produces, by asexual processes such as budding or fission, the
bell or medusoid forms which are sexual. These may remain
attached or become free swimming. They produce ova or
spermatozoa, or both, and from the sexual union of these
elements the non-sexual tubular polyp is again produced. This
regular alternation of sexual and sexless individuals is known
as alternation of generation. In some forms, however, the
polyp has no corresponding bell (as in hydra; corals; sea-
anemone}, and for some bells (as in some large pelagic me*
dusae) there is no corresponding polyp stage.

219. The nutritive processes in the Crelenterata are
marked by relative simplicity. Food, consisting mainly of
small organisms and organic debris, is taken into the mouth
often with the assistance of tentacles. The tentacles are fre-
quently armed with numerous special cells in which are de-
veloped capsules containing long stinging threads, with barbs
or poisonous tips. These may be everted and possibly their
action brings about a partial paralysis of the prey. They

1 See Text-Book of Zoology, Parker and Haswell, Vol. I, p. 127, Fig.

CCELENTERATA.

173

serve also as organs of defense (Fig. 81). Digestion and
circulation both take place in a general cavity (gastro-vascu-
lar) lined with entoderm. In other words the circulatory

FIG. 81.

c

-nu.

FIG. 81. Nettling cells of Hydra (after Schmeil). A, unexploded; B, exploded, b,
barbs; c, the nettling cell in which the nettling organ is developed; en., the cnidocil or
"trigger"; cp., the capsule or nettling organ; f, the nettling filament or lasso; n, neck
of the capsule; nu., nucleus of the cell.

Questions on the figure. Compare the parts of the nettling organ
before and after explosion and note the difference in position. How would
the barbs in the neck of the capsule behave as it is forced inside out by
the compression of the capsule? Find from your reference literature the
nature of the fluid secreted on the inside of the lasso.

ZOOLOGY.

function in this group is not differentiated from the digestive.
In the colonial forms the gastro-vascular cavity of the various
polyps in the colony may be directly continuous (Fig. 85).
In the medusa, the corals, and forms like anemone, the cavity
ic? much more complicated than in the tubular hydroids, on
account of the mesenteries. The entoderm seems to take up
food from the gastro-vascular cavity, in part at least, by means
of the amreboid action of some of the entodermic cells.
Pseudopodia are formed, and particles are directly taken into
the body of the cell. Special gland cells also occur in the
entoderm, by the secretions of which the food undergoes
changes preparatory to absorption. There is no anal opening.
Undigested remnants are eliminated at the mouth. Respira-
tion the exchange of carbon dioxide for oxygen takes place
by means of the individual cells of the body layers, though it
is probable that it takes place more satisfactorily in the thin-
walled, more actively moving tentacles. Excretion is like-
wise a general body function.

220. Motion. All the Ccelenterata are supplied with con-
tractile fibres. Many of these are modified ectodermal or
entodermal cells rather than true mesoderm (Fig. 79, B).
The fibres run both longitudinally and transversely. In the
more active types cross-striate fibres may occur. The attached
(polyp) forms have well-developed longitudinal fibres in the
body-wall and the mesenteries, which enable the soft parts
of the animal to be drawn close to the supporting object. In
the medusoid types locomotion is effected by rhythmic con-
tractions of the bell as a whole. By this means the water is
expelled from the cavity of the bell, and the reaction forces
the animal forward.

221. Support. The attached colonial forms (corals, sea-
fans, etc.) usually possess a skeleton of calcareous or horny
matter commonly secreted by the ectoderm. Each polyp con-
tributes a portion to the common skeleton the corallum. The
corallum differs greatly in form in the different species. This

CCELENTERATA. 1 75

depends on the law of budding or non-sexual reproduction
of the polyps, and the activity shown by the individual in
secreting. In some cases single polyps produce a skeleton (cup-
corals}. The coral reefs of tropical seas are illustrations of
the power of corals to form and excrete carbonate of lime.
Much of the lime-stone of the earth's crust shows that corals
assisted in its formation.

222. Sensation. The nerve cells may be scattered diffusely
over the surface of the body with a mesh-work of fibrils to
connect them with the muscular and nettle cells and with each
other, as in Hydra. In some other polyp-forms there is more
differentiation of cells and fibres, but the elements are still
scattered. In the more active types there is a collection of
the cells either as a connected ring, or in groups, in the ten-
tacle-bearing rim of the animal. Associated with this collec-
tion of the nervous material into a kind of nervous centre,
there are often special areas of sensory epithelium, or sense
organs, developed from the ectoderm. It is not wholly clear
what kinds of stimuli they are suited to receive although they
are designated as " eye spots," or as " auditory " or " olfac-
tory " pits. The tactile sense is undoubtedly present and the
chemical sense (taste or smell), although no special organs
are apparent. Otocysts (see 108) are found in the cteno-
phores and in some medusae, and apparently function chiefly as
organs of equilibration.

223. Reproduction and Development. The occurrence
of both sexual and asexual methods of reproduction has al-
ready been mentioned (218). It is by the latter method that
colonies are normally produced and a given locality well occu-
pied by the species. By means of the sexual method dispersion
is effected, and new regions are occupied. The ova and sper-
matozoa develop in special gonads (ovaries or testes) derived
either from the ectoderm or the entoderm. The sexual cells
usually escape into the gastro-vascular cavity and reach the
outside by way of the mouth. As a rule the sexes occur in

176

ZOOLOGY.

separate individuals. After fertilization cleavage is total but
sometimes not equal. A blastula is formed which is often
converted into a peculiar, free swimming, ciliated larva
(planula), consisting of a two-layered sac with no opening.
This condition may arise by the closing up of an ordinary two-
layered gastrula (as in Aurelia). In other cases the entoderm

o

FIG. 82. Diagrams illustrating development in some of the hydroid types. A,
blastula in which the entoderm (ent.) is produced by proliferation from ectoderm (ect.),
B, ciliated planula formed by the continuance of this process. A split in the entoderm
furnishes the beginning of the gastrovascular cavity (g) of the adult. C, more
mature condition, in which the planula has become fixed: /, foot or attached end;
o, oral or free end at which the tentacles and mouth will be developed.

Questions on the figures. How does this blastula differ from the
typical blastula in the formation of entoderm? What is a planula? Is a
gastrula formed? After an opening forms at the oral end what likeness
is there in the adult to a gastrula? What changes would C need to
undergo to become essentially similar to Hydra?

may be formed by cells budding into and finally lining or even
filling the segmentation cavity of an ordinary blastula (Fig.
83), resulting in a quite similar condition. The planula after
a brief free life becomes attached by one pole and becomes
elongated; a mouth surrounded by tentacles is formed at
the other. . Thus it assumes the typical polyp form. In nearly
all species the polyps may produce new individuals by buds
either from the wall of the polyp or from special organs
(stolons, or runners}. If, when these are mature, they sepa-
rate from the parent no colony is formed. More commonly

CCELENTERATA. 1 77

the daughters remain in association with the parent. The
medusoid individual, often of a very much simpler type than
that described above ( 218), may be produced in a similar
way from a bud. It usually breaks its attachment with the
parent stock and becomes free-swimming.

224. Classification. The following classes of Ccelenterata may be
recognized.

Class I. Hydrozoa. Hydrozoa are Coelenterates with two cell-layers
(ectoderm and entoderm), between which there is a supporting layer (the
mesoglcea) non-cellular in structure. The reproductive cells arise chiefly
from the ectoderm. The life cycle may consist of polyps alone (Hydra) ;
or of medusae alone; or of both in one life history (Campanularia, Pen-
naria, Obelia). Medusoid forms may be free or attached. The gastro-
vascular cavity is not divided by mesenteries. Here are included all the
rather scarce fresh-water ccelenterates, many tubular marine forms some-
what similar to Hydra, and the much diversified colonies of the Siphon-
ophora (as the Portuguese Man-of-War, found in mid-ocean, especially
in the region of the Gulf Stream). See Figs. 84, 85.

Class II. Scyphozoa. Coelenterates in which the mesenchyma con-
tains cellular elements. The reproductive cells arise from the entoderm
and escape into the digestive cavity. Chiefly medusoid forms, though in
some the bell-form alternates with a polyp stage. Types : Aurelia and
the larger jelly-fishes. The majority of the Scyphozoa swim on the sur-
face of the ocean; some are found at considerable depths. Many of
them are very large and handsome. An especially interesting fact in
connection with the development of such a type as Aurelia is that its
polyp (known as the Scyphistoma) is intermediate in its characteristics
between the polyps of the Hydrozoa and those of the Actinozoa. The
Scyphistoma has four ridges which partly separate the gastro-vascular
cavity as do the mesenteries in the Actinozoa.

Class III. Actinozoa, Coelenterates with only the polyp form. Cells
in the mesenchyma. There is a well-developed ectodermic gullet (sto-
modaeum). The gastro-vascular cavity is more or less completely divided
into chambers by mesenteries. Sexual cells entodermal. A skeleton of
calcareous or horny material often present.

Types : Sea-anemones ; sea-fans and corals. The sea-anemones or
sea-roses are common on rocks and other objects just below low-water
mark. Though attached, they have some power of gradually changing
their position. Species of sea-anemones are known in which the indi-
viduals are as much as two feet in diameter, though polyps of the colonial
forms are usually very small.

Class IV. Ctenophora {"comb-bearers"}. The Ctenophora are free-
swimming, pear-shaped jelly-fishes, never occurring in colonies, and not
associated with a polyp stage. They bear eight rows of vibratile plates
composed of cilia, which function as locomotor and possibly as respiratory
'3

i 7 8

ZOOLOGY.

organs, and suggest the name of the group. There is a well-developed
stomodaeum. The gastro-vascular canal branches from this and is much
divided, one division lying under each row of combs. There are two
small aboral openings to the digestive canal known as excretory pores.
The mesenchyma is well developed.

225. Notes on Coelenterates. The food of Coelenterates
consists largely of organic debris broken up by the waves, and
of small animals and plants captured by the tentacles. The
attached forms flourish best in the comparatively shallow

FIG. 83.

-r.

FIG. 83. Hydractina Echinata, after Hincks. c, the ccenosarc, forming an incrusta-
tion over the object on which it lives; n, nutritive polyps; r, reproductive polyps, bear-
ing buds in which are ova; t, tentacles.

Questions on the figure. How many types of individuals seem to be
represented? What evidence of budding do you see in the species? What
is the ccenosarc? What is its nature in Hydractina? What can you find
concerning the habits of the members of the genus ? How does this colony
compare with that in Fig. 84?

water near the shore. Food is especially abundant in such
regions and hence the passive animals are more successful
here than elsewhere. Hydractina ( Fig. 83 ) and even the sea-
anemone form interesting partnerships with the hermit-crab.

CCELENTERATA.

179

The polyps cover up the shell occupied by the crab, thus con-
cealing it from its enemies and its prey. In return the polyps
doubtless profit by a share of the food broken to pieces by the

FIG. 84.

FIG. 84. Physalia, the Portuguese Man-of-war. After Agassiz.

Questions on the figure. For what is this animal remarkable? To
what group of ccelenterates does it belong? Compare Huxley's figure of
the same animal (see Parker and Haswell's Zoology, Vol. I, p. 152, and
other reference texts). What various types of polyps are represented in
the colony? Compare with Fig. 85.

crab, as well as by the change of place as the crab moves about
in search of food. Some anemones have living algae in their
entoderm cells which seem to help supply the animal with
oxygen in return for foods of other kinds.

i8o

s.b

rz>

FIG. 85. A very diagrammatic and generalized illustration of a complex coelenterate
colony. The shaded portion represents the gastro-vascular cavity. The light portion, the
body tissues, b., a bell-like individual developed into an air-bladder; m., mouth; n.s.,
nutritive individual; p.s., protective individual; r.z. 1 , r.z. 2 , r.z. 3 , different types of repro-
ductive individuals; s.b., swimming bell; t, tentacles, which are sensory and protec-
tive structures. After Lang.

Questions on the figure. What is meant by " generalized " above ?
How does such a polymorphic colony as this differ from a highly organ-
ized individual? In what respect is it similar to an individual? What is
the function of the gastro-vascular system? What is the gain in its wide
distribution through the colony? How do the siphonophora differ from
the other colonial ccelenterates ?

CCELENTERATA. 1 8 1

Many interesting experiments have been performed on
members of this group illustrating the power of regenerating
lost parts. Many of the polyps have been shown to have this
power and even the medusae may become perfect animals again
after having lost very considerable portions of their structure.
Hydra, one of the simplest members of the group, is most
famous for its power of regaining its original form, no mat-
ter to what sort of mutilation it has been subjected. As long
as there is a piece of the trunk of appreciable size containing
both ectoderm and entoderm it may regenerate the whole
animal, stalk, mouth, tentacles, and all, under favorable con-
ditions.

Nothing about the Coelenterates is more interesting to the
zoologist than the way in which the individuals in the poly-
morphic colonies (as in the Siphonophora) come to do the
work done by special organs in the higher Metazoa.

226. Supplementary Studies, for field and library.

1. Make a list of all the places where Hydra may be found
in your locality.

2. Can you find an account of any other fresh- water
Coelenterata ?

3. What facts can you find concerning the power of re-
generation in Hydra or other Coelenterates ?

4. Coral reefs: kinds and mode of formation. Conditions
of life necessary to the reef- forming corals.

5. Polyp colonies. Show, by reference to all the speci-
mens and figures you can find, where the newest bud appears
and how this helps determine the shape of the colony.

6. Polymorphism and division of labor in polyp colonies.

7. Corals in geological time.

8. Sense organs among Ccelenterates.

9. Alternation of generation in Obelia. In Aurelia.

10. The symmetry of the Coslenterates.

l82 ZOOLOGY.

11. The structure, position and uses of the nettling cells in
the phylum.

12. Study the polyp of Aurelia (Scyphistoma) from de-
scriptions and cuts, and show in what respects it seems to
stand intermediate between the Hydrozoa and the Actinozoa.

CHAPTER XIII.
UNSEGMENTED WORMS (FLAT-WORMS, THREAD-WORMS, ROTIFERS,

POLYZOA, ETC.).

227. It seems desirable, for the sake of convenience and
in order to prevent a confusing array of details, to embrace
under this head a number of groups of animals which do not
have very much in common except their place of uncertainty
in the animal kingdom. They are not to be considered as
forming a phylum of animals, although in the past they have
often been included by authors with the Annulata (Chapter
XV) under the head of Vermes. There is abundant evidence
indeed to enable one to believe that four or five distinct phyla
are here included. Each of these groups, however, has mem-
bers which bear more or less striking resemblances to animals
belonging to the recognized phyla, especially to embryonic
stages of them. These facts render them of the greatest pos-
sible interest to the zoologist, because they furnish grounds
for the hope that, through the study of this heterogeneous
assemblage, the origin and kinships of all the other phyla may
be made more clear. The same facts make them unfit objects
for extended study in elementary classes.

228. Points of General Resemblance. In external form
these animals differ very greatly. They may vary from a
cylindrical or even a globular form to a thin ribbon-shape.
They agree for the most part, however, in having a main
axis which in the free-swimming forms is usually horizontal
in position, the anterior end of which is structurally distin-
guishable from the posterior. There is usually a distinct
bilateral symmetry (see 116) which takes the place of the
radial symmetry found in the Crelenterates. In some types of
the Ccelenterates there are certain suggestions of bilateral
symmetry but never to the complete exclusion of the radial.

183

1 84 ZOOLOGY.

For the first time is found an assemblage of multi-
cellular animals whose individuals move with one end con-
tinually foremost and one of the body surfaces continually
up and the other down. This is a distinct gain in organization
and accompanies a more active life. The Polyzoa are at-
tached in adult life and have lost this symmetry, and many
of the Rotifers, while having definite anterior and posterior
ends, have lost their right-left symmetry in part, but the em-
bryonic stages of these are in many respects similar to the
more typical forms. By saying that these animals are un-
segmented it is meant that in a distinct individual there is not
usually a linear series of equivalent body-parts or metameres.
There are however several types which reproduce new in-
dividuals by transverse division ("fission"). These new
individuals may remain together, temporarily at least, in a
chain, as in Microstomum (Fig. 89) or the tape-worm (Fig.
91), forming a strobila. In this condition there is a repeti-
tion of all the essential organs in each of the " segments."
Some authors regard this process of strobilation as the con-
dition from which the ordinary segmentation, as seen in the
Annulata, has arisen, by the adhesion and gradual differen-
tiation of the originally similar individuals. The animals of
these groups agree in the fact that the third or mesodermal
layer of tissue becomes more important than it is among the
Coelenterates. In addition to this the mesoderm often, though
not universally, splits, forming a coelom or body cavity
(56) wholly separate from the digestive tract. The ccelom
is lined with mesoderm. All the animal phyla above the
Coelenterates possess this character in some measure and on
this account are called coclomata. These animals further agree
with those above them in the scale of development in possess-
ing a system of excretory tubules which connect the ccelom,
or the mesodermal tissue if there is no ccelom, with the out-
side world. This is sometimes spoken of as the " water-
vascular " system to distinguish it from the blood vessels.

UNSEGMENTED WORMS. 185

229. Laboratory Exercises. An extended laboratory study of these
groups is not desirable, yet the teacher should secure enough material
representing the various included phyla to enable the student to justify
the separation of these uncertain forms from the more exactly defined
phyla; and to show him how ill-defined is the assemblage which we have
thus brought together. The Tape-worm of man may sometimes be secured
from physicians, and other species of Tania are found not infrequently
as intestinal parasites in cats, dogs, or other animals dissected in the
laboratory. The general form, the method of attachment to the host, the
progressive development of the proglottides or " segments ", and the dif-
ference between these segments and those of the Earth-worm should be
noted. Permanent whole mounts of a mature proglottis may be made,
showing the embryos in the uterus. Demonstrations of the structure of
the proglottis may be given by properly prepared transverse sections, if
the equipment and time allow.

An hour's work may profitably be devoted to the study of some one
or more of the common Rotifers, which may be found in water taken
from the stagnant pools in which there is much decaying matter. They
are microscopic animals and are to be recognized by the possession of
discs at the anterior end, which present the appearance of rotating wheels
because of a rhythmic action of the cilia. Make sketches showing the
change of shape which the animal undergoes. How is the change effected?
How is locomotion accomplished? What evidences have you of its ability
to receive stimuli and to respond to them ? How does it get food ? Can
you trace the digestive tract in the body of the animal ? Notice the con-
tracting object just back of the mouth. What conclusions do you reach
as to its function? Give your evidences. Verify by consulting some text-
book. Can you prove from what you see that this is not a single-celled
animal like Stentor? The student should be cautioned against taking
these specimens as closely typical of the whole group of Rotifers, since
there is very great variety of form among them.

Planarians often appear in the laboratory in water containing an
abundance of decomposing organic matter, taken from ponds and foul
streams. The most important points to be noticed are their general form,
the method of locomotion, sensitiveness to stimuli, and life habits. Non-
sexual reproduction by fission is frequent among them.

The Polyzoa occur as tufts of many minute animals in colonies at-
tached to objects in the water. Plumatella is a rather common fresh-
water form and makes a beautiful demonstration to illustrate the ordi-
nary physiological processes, as motion, feeding, the action of the diges-
tive tract in churning the food, sensitiveness to stimulus and the like.
Schools near the sea-shore will find an abundance of marine material for
the comparison of the colonial forms of different species of Polyzoa, since
they are more common in salt than in fresh water.

230. Classification and Description. Phylum Platyhelminthes (Flat-
worms'). In the worms of this phylum the body is flattened or com-

i86

ZOOLOGY.

pressed in a dorso-ventral direction, and from this fact the name is given.
They are soft-bodied animals without any true skeleton. There is no
body cavity and no true blood-vascular system. The space which would
be given to such structures is filled with a spongy connective tissue.

FIG. 86.

FIG. 86. Diagram ot a Turbellarian, showing the general arrangement of the nervous
structures and one of the modes of occurrence of the excretory tubules, which in this
case open separately into the pharynx, on the ventral side of the animal, b., brain; e,
eye-spots; ex, excretory canals consisting of a transverse portion passing from the
mouth toward the dorsal side (see also Fig. 87), and longitudinal tubes which branch
into the capillary vessels terminating in f, the flame cells; lc., lateral nerve cords; m,
mouth.

Questions on the figure. Compare this figure with the next and
identify the structures shown in both. What other positions of the mouth
do you discover in the Turbellaria, as figured in reference texts? What
other arrangement of the excretory canals and pores?

Through this body-mass run the minute tubes of the excretory or water-
vascular system (Fig. 86, ex.}, often terminating internally in special cells
(Home cells, Fig. 88). These tubes have external pores. By means
of this system of organs waste products, probably of a nitrogenous nature,
are eliminated from the tissues. The digestive tract may be wholly want-
ing as in the Cestodes, or a simple or forked sac, or a central sac with
lateral branches. It is blind, *. e., has only the oral opening. In the more
complicated types of stomach the much-branched sac serves the function
of carrying the digested food to all parts of the body. Many of these
forms are parasitic and in consequence the organs referred to are often

UNSEGMENTED WORMS.

187

very much simplified and degenerate. The digestive tract, for example,
may be entirely lost. Reproduction by transverse division is not uncom-
mon. By this method strobilae or chains of more or less closely connected
individuals occur. The sexual organs are exceedingly complex, particu-
larly in the parasitic members of the group (Fig. 92). The develop-
ment is in some instances direct, in others indirect. The principal classes
are the Turbellaria, Trematodes and Cestodes.

FIG. 87.

FIG.

FIG. 87. Diagram of transverse section of a Turbellarian through the region of the
mouth, d.m., dermo-muscular wall containing longitudinal fibres; ex, excretory system;
f, flame cells; g, gut; I.e., lateral nerve cord; tn, mouth; m.f., muscle fibres; ph.,
pharynx; t, testis; u, uterus; y, yolk glands.

Questions on the figure. Determine with care the relation of this
to the preceding diagram and identify the common structures. What new
structures are represented here? What would be their position in the
former figure? The great range in position of the muscle fibres and the
spongy character of the body contribute to what powers?

FIG. 88. Diagram of flame cell, the internal terminus of the excretory tubules, c,
cilia lining the tubule; f, special cilia constituting the -flame; n, nucleus of flame cell;
p, cell processes; v, vacuole or cavity in cell communicating with the capillary
tubules (().

Questions on the figure. What is the function of the cell itself?
Of the flame?

Class I. Turbellaria (Planarians, etc.}. These are mostly small non-
parasitic Platyhelminthes with a ciliated ectoderm. They are chiefly
aquatic and are carnivorous. The ventral mouth may be anterior, pos-
terior, or median in position. It opens into a muscular eversible pharynx,
which may be used to assist in locomotion. The digestive tract may be
simple or very much branched. The brain consists of a pair of ganglia
in the anterior region. From the brain lateral nerve cords pass backward

i88

ZOOLOGY.

through the body. The excretory organs (Figs. 86, 87) usually consist of
two or more longitudinal tubes which open on the exterior separately or
by a common orifice. The position of the opening varies very much in
the different orders. The tubules are much branched interiorly and pene-
trate the soft tissues of the body as minute capillaries with thin walls.
They terminate in cells of special structure which are excretory in func-
tion. A group of cilia (the flame, Fig. 88, /) helps in creating a current
in the capillary tubes. The lining of the tube may also be supplied with
cilia. The Turbellaria have remarkable powers of regenerating lost por-
tions. Experiments show that very small portions of an individual will,
under favorable conditions, reproduce all the parts of a complete animal.
In habit they may be terrestrial, fresh-water or marine. They vary in
size from microscopic fresh-water forms to a length of six inches or more
in the case of the marine and land types (Figs. 86-89).

FIG.

FIG. 89. Diagrammatic sagittal section of Microstomum, showing a chain of four
zooids produced by fission, b, brain of the original zooid (the exponents indicating
corresponding structures of the more recently formed zooids) ; c, ciliated pit; d, dis-
sepiments indicating different stages in the separation of the zooids; e, eyespot; ent,
entoderm; g, gut; gl., glandular cells about the mouth; m, mouth of the original worm.

Questions on the figure. What various evidences can be found of
the relative age of the zooids? Is the mouth formed apparently from
entoderm or ectoderm? Is the gut a blind sac? What incidents seem
necessary when this chain separates at the oldest plane of division, and
forms two chains, in order that each may be like the parent? How is
this like segmentation in annulates (see Fig. 99)? How unlike?

Class 2. Trematoda. 'The Trematodes are small, usually parasitic,
Platyhelminthes. The ectoderm is provided with a protective "cuticle"
and is consequently destitute of cilia. They possess a well-developed and
often much-branched digestive sac, which has only one opening the
mouth. Usually one or more sucking discs are present. By means of
these the parasite attaches itself to the host. The nervous and excretory
systems are similar in general to those of the Turbellaria, but are some-
what better developed and more complex. In those members of the class
which are external parasites there is usually no metamorphosis in the
development. In the internal parasites, as the Liver-fluke of the Sheep,

FIG. 90.

A series of diagrams illustrating the life cycle in the LIVER FLUKE (Distomum) .
After Thomas, Leuckart, and others. A, egg in its case; B, early embryo, still in
case; C, free-swimming ciliated embryo; D, same after encysting in tissues of snail
(sporocyst) ; E, sporocyst at later stage producing by internal, non-sexual processes
new sporocysts, and redia (r~) which break from the sporocyst and lead an inde-
pendent life of their own in the tissues of the snail; F, a mature redia producing
within itself new generations of rediae, and a new type of larva, cercariee which
escape by a birth-pore (b.p.~) and make their way into the water; G, cercaria; H,
same after losing its tail and becoming encysted; /, the young fluke in the liver of
the sheep, where it becomes sexually mature and produces perhaps 500,000 new
eggs, b, brain; b.p., birth pore; c, cercaria; c.m., cell masses, embryos formed non-
sexually within sporocysts and redise; e, eye-spots; ex., excretory tubules and pore
(only the posterior portion shows); g, gut; m, mouth; ph, pharynx; r, redia; s,
suckers; sc, sporocyst; +, stages in which non-sexual reproduction occurs; *, stage at
which sexual reproduction occurs.

Questions on the figures. In which stages are eyespots found? Num-
ber and position of the suckers? In which stages found? What is the
result of increasing the points at which reproduction occurs in the cycle?
Is this a combination of metamorphosis and alternation of generation?
Your reasons for your answer? Compare this with the life history of the
tape-worm. Note the encysted stage by which it passes from water to its
host in each instance.

,89

ZOOLOGY.

there is frequently a most complicated metamorphosis coupled with an
alternation of sexual and non-sexual generation (see 218). A
Liver-fluke (Distomum hepaticum} is found in the bile ducts of the liver
of the sheep, where it gives rise to a much-dreaded disease " liver rot."
The eggs which are formed, fertilized and pass through the early stages
of cleavage here, pass out of the bile ducts to the intestine and thence to
the exterior. If the larva reaches water it develops into a free-swim-
ming larva (Fig. 90, C.), which to insure further development must bore
into the tissues of a particular pond-snail (Limncea truncatula). It there
develops into a kind of sac (sporocyst) from the inner cells of which
special cells are budded (Fig. 90, ). These cells have the power of
developing into embryos of a second generation by cell division that
is to say, non-sexually. Several such non-sexual reproductions may occur
in the body of the snail (Fig. 90, -(-). These later generations of larvae
pass, often by the death of the snail, into the water, whence they may
enter the alimentary tract of the sheep in drinking. The larvae find their
way to the liver and develop there again into the adult fluke. It is evi-
dent that such a form must have immense powers of reproduction, when
it is considered that the reproduction takes place at several points in the
life cycle (Fig. 90, -j- *). This may be seen to be a necessity to compensate
for the great loss of life involved in changing from host to host. It is
said that a single fluke may produce half a million eggs. Each of these
which succeeds in reaching the host snail may produce hundreds of the
last generation of asexual individuals. The disease is prevalent only in
those countries where this species of Limncea occurs. It is much worse
in wet years. Millions of sheep have died in England alone, in a single
year, from the attacks of this parasite. Trematode parasites are common
among the vertebrates and frequent most diverse organs.

Class 3. Cestodes (Tape-worm, etc.}. The Cestodes are internal
parasites having a complicated life history usually involving two hosts.
In the tissues of the first host occurs the " bladder-worm," Cysticercus,
or embryonic stage (Fig. 91, A) ; in the intestine of a second host the
strobila or adult tapeworm (Fig. 91, C) is found. The adult form has
no mouth or digestive tract, the animal taking its food by absorption of
the digested material in which it is bathed. The anterior end is supplied
with hooks or suckers by means of which it attaches itself to the intes-
tinal wall. Just behind this " head " is a region in which transverse
division (Fig. 91, z; and 122) is continually going on. This results
in the continuous formation of new segments or proglottides, the older
ones being pushed further from the head by those newly formed. Each
proglottis becomes in time a sexually mature hermaphrodite individual.
All stages of sexual maturity are found in one strobila or colony, the
posterior individuals being most mature. At the posterior end of an old
colony the proglottides (Figs. 91, 92) are filled with the developing embryos,
and on breaking away from the chain these brood cases pass with the
faecal matter from the intestine. In this way it becomes possible for the
embryos to find the way into a new host. On being swallowed by some

UNSEGMENTED WORMS.
FIG. 91.

D

FIG. 91. Diagram showing some stages in the life history of the Tapeworm (.Tasnid).

A, Cysticercus or Bladderworm stage, before the "head" protrudes from the bladder;

B, same, later stage; C, Strobila, or chain of proglottides, many being omitted; D,
embryo, such as fill the uterus of the mature proglottides. It is protected by a shell.
b, bladder; ex., excretory canals; g, genital pore; h, head or scolex provided with hooks
and suckers (.s) ; , uterus in a mature posterior proglottis; s, zone of budding or seg-
ment formation. The numerals show the approximate number of the segments, reckon-
ing from the front. Not more than 5 per cent, of real length of the chain is
represented.

Questions on the figure. What arguments do you find from the
figur.e for considering the strobila an individual? What for considering it
a colony? Where does non-sexual reproduction occur? Where sexual?
Seek figures of stages between D and A in the reference books.

192

ZOOLOGY.

suitable animal they break from their cysts, bore through the wall of the
digestive tract into the tissues. Here they grow, become encysted and at
this stage develop, in anticipation of the needs of the adult worm, the head
or scolex which remains attached to the bladder-like cyst (Fig. 91, A, B).
Development stops at this point unless the flesh of this host is eaten by

FIG. 92.

FIG. 92. Diagram of a sexually mature proglottis of Tania. A, anterior end; e,
embryos; ex., excretory canals; g.p., genital pore; ov., ovaries (paired); r.s., recep-
taculum seminis; s.g., shell gland; t, testes; ut., uterus filled with embryos; v, vagina;
v.d., vas deferens; y.g., yolk gland.

Questions on the figure. Why is self-fertilization possible in tape-
worm? What is the function of the various portions of the reproductive
apparatus? Trace the following steps and indicate where each incident
happens : formation of eggs and sperm ; passage of sperm to vas deferens
and into vagina ; storing of sperm in receptaculum seminis ; fertilization
in the oviduct; addition of yolk; ovum covered with the shell secretion;
passage into uterus where development proceeds.

some other animal. When this happens the bladder is thrown off, the
head becomes attached to the wall of the intestine of the carnivorous host,
and the active formation of the chain of proglottides begins again. The
more common Tape-worms of man are Tcenia solium and Tcenia saginata.
The former is more common in Europe and is received into the system

UNSEGMENTED WORMS. 193

by eating the raw flesh of the pig, in which the bladder-worm stage occurs.
The latter is obtained chiefly from beef and is more common in America.
Only by adequate cooking is the danger of infection removed. The Amer-
ican habit of eating beef rare contributes to the spread of the pest. Other
tape-worms infest, as their double host, the dog and the rabbit; the cat
and the mouse; the shark and other fishes. The excretory system is a
pair of continuous lateral tubes with transverse connections in the various
proglottides (Fig. 92, ex). The nervous system in the adult tape-worm
includes a rather complex series of loops containing nerve-cells, in the
scolex, with right and left lateral lines of nervous tissue running the
length of the strobila. There are numerous longitudinal, transverse
(circular), and dorso-ventral muscle fibres passing through the spongy
tissue of the worm. There is a well-developed external cuticle which
helps protect the animal from the action of the digestive juices of the
host.

Phylum Nemathelminthes (Round- or Thread-worms). Nemathel-
minthes are elongated, cylindrical forms which taper at the ends. The
body is covered by a dense cuticle. Some are aquatic but most are para-
sitic at least during a part of their life. An alimentary tract is present
and has both a mouth and an anus. There is a coelom which is not
divided into chambers and contains a fluid without corpuscles. There is
no circulatory system other than this. There are no special respiratory
organs. The central nervous system consists of a ring around the oesoph-
agus. This contains some nerve cells. From this ring nerves arise at
various points and pass both forward and backward. The chief posterior
nerve is ventral, but there may be also dorsal and lateral ones. The sexes
are usually separate. Development is sometimes direct, sometimes in-
direct. The best-known representatives arc the round-worms (Ascaris),
different species of which are found in the intestine of man, of the pig,
and of the horse ; vinegar-" eels " ; trichina.

Trichina is one of the most dangerous of the nematode parasites. The
sexually mature worm occurs in the intestine of the rat, the pig, man, or
other mammal. The young are retained by the mother in the uterus
until well developed. When born the young bore through the wall of
the intestine of the host and make their way to the muscles, where they
become encysted and cause degeneration of the muscle fibres and often
other acute symptoms of the disease known as trichinosis. The larvae
remain in their cysts indefinitely or until the death of their host. For
further development the flesh must be eaten. In the intestine of the new
host where the cyst is dissolved the adult condition is quickly reached,
reproduction takes place again, the embryos migrate into the muscles and
the new cycle is begun. We do not find here the non-sexual reproduction
that helped make the Liver-fluke so prolific, but the reproductive power
of Trichina is very great without this. It is estimated that an ounce of
" measly " pork may contain 80,000 cysts of Trichina, and that each
feniale produced from these embryos may contain at one time 1,000 or
more embryos. During her life she may produce ten times this number.

ZOOLOGY.

Thus the 40,000 females from such a meal would soon supply 40,000,000
young worms for the infection of the muscles, with the ability of renew-
ing the supply at short periods. Perfect cooking is the only sure safe-
guard against the possibility of infection.

FIG. 93.

FIG. 93. Diagram of a sagittal section of a Rotifer, b, brain; bl., excretory
bladder; c, cloaca, the common opening of digestive and reproductive organs; co,
coelom; e, eyespot; ex, excretory canal; /, flame cells; f.g., foot gland; ft., foot; g, gut;
m, mouth; m.f., longitudinal muscle fibres; mx, mastax; o, ovary; ph., pharynx; s.g.,
salivary gland; t, tentacle; tr, trochus, or cilia-bearing disc.

Questions on the figure. What sets of organs and functions are in-
dicated in the diagram? Does this seem a lower or higher form than the
other types studied in this chapter? What are your grounds for your
answer? What indications of segmentation are represented in the figure?
Is the mastax in the stomodaeum or mesenteron? Where do the various
authors classify Rotifers ?

UNSEGMENTED WORMS. 195

Phylum Trochelminthes {wheel-worms or rotifers'). The Rotifers
or wheel-animalcules are microscopic animals. They are usually bilater-
ally symmetrical. The anterior end possesses a retractile disc supplied
with cilia variously arranged, the rhythmic motions of which often give
the appearance of a rotating wheel. From this the name of the group
comes. This organ assists in locomotion and produces currents in the
water by which food is brought within reach of the mouth. There is a
digestive tract with both mouth and anus. The pharynx into which the
mouth opens is provided with a chitinous grinding apparatus (mastax).
Usually a pair of digestive glands open into the stomach. The nervous
system is usually limited to a single ganglion dorsal to the pharynx.
Eye-spots and other sense organs, called tactile rods or antennae, are pres-
ent. There is a true ccelom communicating with the exterior by means
of excretory tubules. For a diagrammatic view of these structures see
Fig. 93-

The sexes are distinct and are frequently very different in appearance.
The males are often much smaller than the females, are much less numer-
ous, and are often degenerate. The summer eggs are of two kinds
large and small and develop parthenogenetically. The large eggs pro-
duce females and the small, males. The winter eggs have a thick shell
and are believed to require fertilization in order to develop. They rest
during the winter and in the spring develop into females. Development
is direct. The adult cofftlition in the Rotifers suggests the larval (trocho-
phore} condition in some Annulata. There are some traces of external
segmentation in the tail or foot region in some species and for these
reasons some authors class the Rotifers near the Annulata. Rotifers are
aquatic, being more common in fresh water than in the sea. They are
abundant in water-troughs, gutters, ponds. They are capable of resuming
activity after having been dried up in the mud for a year or more. This
power must be of great value in preserving the species as well as in
spreading it.

Phylum Molluscoidea (mollusk-like) . The two groups' included here
are quite diverse in general appearance and habit. Their larval stages
have more points in common than the adult. There is in the adult a
variously-shaped tentacle-bearing ridge (lophophore) about the mouth.
The central nervous system consists of one or two ganglia about the
oesophagus. They have often been grouped with the mollusks but authors
are agreed that much of the seeming resemblance to mollusks is super-
ficial.

Class i. Polysoa (Bryozoa; sea-mats; corallines). The Polyzoa are
colonial animals which resemble in general appearance some of the com-
pound hydroids. The individual animals however are very different in
their structure. They are found both in salt and fresh water. In Poly-
zoa (Fig. 94) the digestive tract is sharply bent, the anus opening close
to the mouth either within or outside the circle of tentacles (lophophore) .
A distinct coelom is typically present. There are no blood vessels.
An exoskeleton is formed by the ectoderm, by means of which the indi-

196

ZOOLOGY.

viduals of the colony are held together. Each member of the colony
may retire into its own particular portion of the exoskeleton, when dis-
turbed, by the contraction of appropriate muscles. The brain consists
of a single ganglion lying between the mouth and anus. The two sexes
usually occur in the same individual. The colonies are formed by budding,
which takes place in each species in a way that is characteristic of that
species. Thus it comes about that the colonies differ as much in general
appearance as their individuals do in structure.

FIG. 94.

Parker and Haswell, after
>dy

Questions on the figure. Is this an individual or a colony? What is
the function of the retractor muscles? To what degree are the polyps
capable of contraction as shown in the figure? The value of this power?
What are the statoblastsf

Class 2. Brachiopoda {arm-footed; lamp-shells). Brachiopods are
marine forms chiefly of geological interest, as there are at present only a
few living species. They were very prevalent in early geological times.
They possess a bivalved shell which suggests that of the bivalve Mollusca
(as the clam). From this external resemblance they have long been
classed as mollusks. The valves are strictly dorsal and ventral in the
Brachiopods, however; whereas in the mollusks they are right and left.
Their shell is therefore no longer considered as homologous with the
mollusk shell. The internal structure is still further removed from that
of the clam. The digestive tract is often bent much as in the Polyzoa,
and the mouth is surrounded by a tentacle-bearing lophophore (the

UNSEGMENTED WORMS. 197

"arms"). The lophophore may have a skeletal support which in differ-
ent types assumes different shapes (loop, helix, or spiral). A peduncle
usually extrudes at the hinge, by means of which the animal attaches itself
to foreign objects. The Brachiopods are not colonial. The student is
referred to the more extended texts for illustrations of this group of
animals.

231. Notes on Ecology and Distribution. The organ-
isms included in this chapter represent the most varied modes
of life. The Turbellarians are free animals and may be ter-
restrial, fresh-water or marine; the Rotifers are as a rule
free-swimming and occur chiefly in fresh water; the Polyzoa
are aquatic, attached, colonial forms but lead for the most
part an independent existence, or may occasionally be com-
mensal with other types of animals; the Brachiopods are
marine and may be attached, but are not colonial ; the Trema-
todes and Cestodes represent all kinds and degrees of para-
sitism. Even if all these classes of animals could be con-
sidered akin, their habits of life and their consequent adapta-
tions are so various as to produce the greatest range of gen-
eral form and special structure.

If we consider the relatively small number of species of
animals in these groups, the species of the Platyhelminthes are
among the most widely distributed of the metazoa. This is
true both of the free Turbellaria and the parasitic Trematodes
and Cestodes. There is probably not a large group of the
metazoa which escapes being the host of one or more of these
worms at some stage of its life history. The fact of para-
sitism, the ability to carry on the life cycle in a series of hosts,
and the prevalence of the carnivorous habit among its hosts
all help the distribution. The organs more commonly infested
by the parasites are the digestive tube, the blood and lymphatic
vessels, the coelomic cavity or other organs where the nutritive
fluids of the body are abundant. They produce all sorts of
disorders from mere functional disturbance (such as digestive
disorders and anaemia from the presence of the tape-worm)
to the destruction of the tissues of the organs involved. It
. is very commonly true that the adult or sexually mature in-

198 ZOOLOGY.

dividuals are produced in one host, and the eggs or larvae pro-
duced by them find their way into another species of host where
a portion of the development toward maturity occurs. The
transfer of the parasite from the second back to the first host-
species is necessary to complete the cycle. In some instances
there is not a change from one animal to another, but merely
from one organ to another in the same animal, as in Tcenia
murina of the rat. In size the unsegmented worms vary
from minute microscopic dimensions to thirty feet in length
in the tape-worm, Bothriocephalus latus. Some suggestion of
their importance to man and the higher animals may be
gathered by reference to the following table (p. 199).

232. Supplementary Studies for the Library.

1. In what different ways are the forms included in this
chapter classified in the various text-books to which you have
access ?

2. Consider the economic importance of the parasites in-
cluded in this chapter.

3. Make a further study of the life histories of selected
representatives of these parasites.

4. Illustrate by means of the unsegmented worms the de-
generation and simplification which attends parasitism.

5. In what various ways do the intestinal parasites in the
group adhere to the walls of the digestive tract of the host?

6. Do you think the domestic animals are more or less
likely to be attacked and suffer from these internal parasites
than the wild? What evidences would you offer for your
view?

7. Prepare for the class a diagram of the reproductive
organs in the Tape-worm, indicating the function of each of
the portions.

8. What is meant by the " dermo-muscular " sac in worms?
Its functions?

9. Report on the importance of the Brachiopods in early
geological time, with the main structural features of the class.

UNSEGMENTED WORMS.

199

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CHAPTER XIV.

PHYLUM IV.-ECHINODERMATA (STAR-FISH, SEA-URCHINS, SAND-
DOLLARS, SEA -LILIES).

LABORATORY EXERCISES.

233. Asterias (Star-fish). Both dry and alcoholic, or
otherwise preserved, materials should be at hand.

1. General form.

Central disc.

Rays; number, form, size, etc. Compare several in-
dividuals.

Oral surface (contains mouth) ; aboral- surf ace. Note
all the differences between these surfaces, both
in the arms and the disc.

The axis of an arm is known as a radius; the space
between is interradial.

Is the body bilaterally symmetrical or radially sym-
metrical? Give the reasons for your conclusion.

2. External anatomy.
Oral surface.

Mouth : position and surroundings.

Ambulacral groove: position, relation to the mouth,

extent.
Ambulacral feet: how arranged? Is the foot hollow

or solid? Pull off one, and examine with lens or

low power of the microscope.
Aboral surface.

Madreporic body: position (radial or interradial?),

shape, size, structure.
Bivium; trivium (see text, 237).
Examine the spines on both surfaces and determine

the arrangement and shape in different regions.

How are they fixed to the body? 1
200

ECHINODERMATA. 2OI

Pedicellarise (at the base of the spines) ; papulae (soft

bodies among the spines). Examine with lens.
Make an outline drawing of each surface, filling in the
details of the disc and one arm and showing the points above
determined. Sketch one of each of the various classes of
spines in profile.

3. Organs of the body-cavity.

Using .alcoholic or other moist preparations, cut into one side of an
arm of the trivium, making an incision from near the tip almost to the
disc. Cut across the back of the arm near the 'tip and make a similar
incision on the other side. Lift the flap thus separated and notice the
organs attached to it. The material should be dissected under water or 50
per cent, alcohol, or kept moistened therewith.
Hepatic caeca ; extent, number, and attachment.

Detach the hepatic caeca from the aboral wall by breaking the mesen-
teries, and treat all the arms of the trivium as above.

Carefully connect the incisions across the interradii and remove the
entire aboral wall except that around the madreporic body and that be-
tween it and the centre of the disc, being careful to disturb none of the
soft parts. If material is scarce the teacher should make a few dissec-
tions to be used as demonstrations.
Notice :

Body-cavity, its extent and contents.

Stomach: pyloric (aboral) portion; shape, position. Are the hepatic
caeca connected with it? Verify. (The stomach opens aborally into
a small, short rectum and anus usually very difficult of demon-
stration.) Rectal diverticula? number and position?
Cardiac (oral) portion of stomach; pouches, number and form; re-
tractor muscles, attached to the floor of the arms.
Mouth; peristome.

Remove the hepatic caeca from one arm and find the genital glands,

which lie in the floor of the body-cavity. What is their number

and arrangement? At what point do they connect with the body

wall? Can you prove that they communicate with the exterior?

Ampullae (on ventral floor) : determine if they connect with ambu-

lacral feet.

Make three diagrams showing the position of the organs thus far
studied : ( I ) the aboral surface with the wall removed, showing
the stomach in the disc, the hepatic caeca in one arm, the repro-
ductive bodies in a second, and the ampullae and retractor muscles
in a third; (2) a transverse section of an arm about midway be-
tween its ends; and (3) a sagittal section of an arm continued
through the disc.

202 ZOOLOGY.

4. Ambulacral system.

In a specimen from which the preceding organs have been removed,
make a transverse section of an arm about an inch from the disc. Find
the radial water canal, a small tube lying just outside the skeleton in
the ambulacral groove. Force air into it with a blow-pipe, or inject a
colored solution with a hypodermic syringe. What other structures are
affected? Trace connection between radial canal, ampulla, and ambulacral
feet. Compare the number of ampullae and the number of feet. Follow
the radial canal toward the disc. How does it terminate?

From the madreporic body trace the S-shaped stone canal toward the
oral surface. How does it terminate?

Ring canal: its position. Are there any other structures (sacs) in
communication with the circum-oral ring-canal beside the stone canal and
the radial water-tubes? form and position?

5. Nervous system.

There is a radial nerve (in the skin) superficial to the radial water
canal in each arm. The radial nerves unite to form a circumoral nerve
ring.

6. Skeletal parts.

Dried material and portions soaked for a day or so in a 10 per cent,
solution of potash should be used to supplement the alcoholic specimens.

Is the skeleton complete, i. e., are the ossicles in contact?

Are they similarly arranged on the aboral and oral surfaces? Which
surface shows the greater differentiation? Illustrate, and find a reason
if you can. How are the ossicles related to the spines? to the papulae?
Study with some care the ossicles forming the ambulacral groove, begin-
ning at the middle line.

Ambulacral rafters : shape and arrangement.

Ambulacral pores; are they in, or between, the ossicles?

Adambulacral ossicles (just lateral to the former) ; how do these
compare in number with the ambulacral ossicles ?

" Cross-shaped " ossicles.

Which of the above bear spines? what kind?

Place some of the cleaned ossicles in dilute hydrochloric acid. Result?
What is the significance of this result?

7. Physiological experiments are possible only near the seashore. The
animals must be kept in sea water, and studied soon after being collected.
When possible, locomotion, the action of the ambulacral feet, feeding, and
sensitiveness should be studied. Do you find any indications, among the
specimens provided, of the power to renew a lost arm? With care and
perseverance, at the proper time of year, the sexual elements may be col-
lected and the maturation, fertilization, and cleavage of the ovum illus-
trated in this group. Teachers in inland schools should procure, when-
ever possible, slides demonstrating the early development of the star-fish
or sea-urchin.

8. Compare briefly the external features of other " stars " with that
already studied.

ECHINODERMATA. 2O3

234. Sea-urchin (Echinus or Arbacia).

A few skeletons of sea-urchins and sand-dollars will be of great value
in enabling the pupil to see how the same general plan of structure may
be varied to meet different needs.

1. Spines (if present) : arrangement and method of attachment. Are
they of the same appearance and composition as the skeleton? Do you
find any signs of the former presence of ambulacral feet? If so what,
and how arranged? Can they all have the same function as in the
star-fish ? Proofs ?

2. Ossicles : Make out the boundaries. Compare with the condition in
the star-fish. What are the special advantages gained by each arrange-
ment? Can you find anything corresponding to ambulacral ossicles?
(Look for the pores.) What corresponds to the ambulacral groove in
Asterias? Identify the interambulacral ossicles. How arranged? What
is radial arid what interradial in the urchin? What in the sea-urchin would
correspond to the oral and aboral surfaces in the star-fish? Evidences?
Find the madreporic body. Make a plot of all the ossicles in this region,
noting the differences. Find the genital pores.

3. "Aristotle's lantern" (the mouth apparatus).

Examine the structure as a whole. How related to the body? Study
the parts in their relation to each other. Number, and method of action?

DESCRIPTIVE TEXT.

235. The Echinoderms form a very distinct group of ani-
mals, which in the adult condition at least show a decided
radial symmetry. They possess a more or less extensive cal-
careous exo-skeleton with outwardly directed spines. The
star-fishes, sea-urchins, brittle stars, sea-lilies, and sea-cu-
cumbers are representatives. They are marine in habit and
may be either fixed or slow-moving. They agree with the
Ccelenterates in having radial symmetry, and in the absence
of a well-marked brain and other signs of cephalization.
There is considerable ground for believing that this is an out-
come of their sluggish habit, since the larval condition is
bilaterally symmetrical, and radial symmetry is clearly adapted
to a passive life. It is difficult to determine the relationships
of the Echinoderms ; yet it seems probable that their ancestors
were bilateral forms. Perhaps they should be considered as
connected with the worms rather than with the Ccelenterates.

236. General Characters.

i. Larvae are bilaterally symmetrical; in the adult there is

204 ZOOLOGY.

a more or less complete radial arrangement of equivalent parts,
usually on the plan of five. In this radial plan all the principal
sets of organs share: as the nervous, digestive, reproductive,
etc.

2. There is a complete differentiation of digestive tract and
body cavity.

3. The blood-vascular system is partially differentiated
from the body cavity, but communicates with it.

4. A calcareous exo-skeleton occurs, derived from the meso-
derm. It may consist of isolated spicules or united plates.
Associated with these are usually spines, from which the
group is named.

5. A water-vascular system, consisting of a series of tubes
(closed except at one point), muscular sacs (ampullae) and
distensible feet, serves a locomotor and respiratory function.

6. Reproduction sexual; development usually indirect, i. e.,
with a metamorphosis. Reproduction by budding does not
occur.

237. General Survey. The majority of echinoderms have
a central disc in which is located portions of the various sets
of organs. Ordinarily there radiate from this disc more or
less clearly defined rays or arms in which lie radial outgrowths
from the central organs. The spaces between the rays (inter-
radii) may be bridged by growth in such a way that the dis-
tinction between rays and disc is not marked. In crinoids the
arms may be much branched. The oral-aboral axis is usually
pronounced, often short, and is vertical in position (asteroids,
echinoids, crinoids, etc.), though in the sea-cucumbers (holo-
thuroids) it is horizontal and much elongated. Star-fish are
flattened vertically, as are the sand-dollars, but many of the
urchins (echinoids) are dome-shaped. The antimeres are at
right angles to this chief axis. In addition to this dominant
radial symmetry, there is seen even in the adult a suggestion
of the bilateral condition. The madreporic body generally
occurs in only one interradius, and a plane passing through
it and splitting the opposite arm divides the body into two

ECHINODERMATA.
FIG. 95-

205

FIG. 95. Starfish, from chart of Leuckart and Nitsche.

Questions on the figure. How would you describe the symmetry
of the animal? Identify and name, by comparison with the diagrams and
the text, all the structures which show in the figure. Compare this with
specimens or figures of the common American species and note the chief
differences.

symmetrical halves. No other plane does this. The two arms
embracing the madreporic body are known as the bivium, the
remaining three, the trivium. In some of the echinoids the
bilateral symmetry becomes much more pronounced than in
star-fish.

238. The integument consists of an outer ectodermal por-
tion which is often ciliated (cilia wanting in the holothuroids
and ophiuroids), and a subepithelial, mesodermic layer in
which is developed the calcareous ossicles. These may occur
as spicules, as rods, or as plates in the various classes. Fre-
quently the ossicles bear spines which may or may not be

2O6

ZOOLOGY.

movable. The spines are useful in defense and locomotion.
Special forms of spines known as pedicellarioz often occur
(asteroids and echinoids). They consist of two- or three-
pronged pincers moved by muscles. They may be mounted
on short stalks. It is suggested that they help clear the body
of foreign objects which lodge 'among the spines.

239. Digestive System. The mouth and anus usually
open at opposite poles of the principal axis (asteroids, holo-
thuroids, and some echinoids). When the axis is vertical the
mouth is usually directed downward, in the centre of the oral
surface, and the anus occupies a more or less central position
on the upper or aboral surface. In some of the echinoids the
mouth or anus, or both, have vacated their central position and
have come to occupy opposite margins of the body. The diges-
tive tract is a simple tube, in the holothuroids running spirally
through the body. In the echinoids a similar condition is
found except that it begins in a complex masticating apparatus

FIG. 96.

FIG. 96. Vertical (sagittal) section through an arm and an interradius of a Starfish
(diagrammatic), a, anus; amp., ampulla; c.b., circular blood vessel; c.w., circular
water canal; co., coslom; co.e., coelomic epithelium; d.b., dermal branchiae; e, position
of the eyespot; ect., ectoderm; ent, entoderm; f, ambulacral foot; g, ambulacral
groove; h, hepatic caeca Jir liver; t, intestine; i.e., intestinal caeca; mes, mesoderm;
mo., mouth; m.p., madrep'oric body; n.r., nerve ring; os., ossicles in mesoderm;
r.n., radial nerve band; r.b., radial blood vessel; r.p., reproductive pore; r.w.,
radial water canal; s.c., stone canal; sp., spines; s, lacunar spaces in the mesoderm.
(Adapted from various sources.)

ECHINODERMATA. 207

of five parts (Aristotle's lantern). In the asteroids the mouth
opens by a short oesophagus into an expanded stomach, which
is divided into an oral, or cardiac, and a pyloric portion (Fig.
96). From the pyloric part the narrow intestine passes to
the anus. Outpocketings (caeca) may occur in any of these
divisions. The most important are the hepatic caeca, which
are glandular in function.

240. The body cavity is usually well developed both in the
disc and in the arms, is lined with a ciliated epithelium, and
contains a fluid with amoeboid corpuscles. It is completely dis-
tinct from the digestive cavity. Thin outgrowths of the body-
wall (papules or branchice) contain extensions of the ccelom.
These assist in respiration.

241. Ambulacral or Water- vascular System. This sys-
tem of tubular organs is peculiar to the echinoderms. It
originates (see also 248), in common with the body cavity, as
an outgrowth from the archenteron and is to be regarded as
a specialized portion of the body cavity. In some cases these
two cavities are in communication in the adult. It consists
essentially of a ring-vessel about the mouth from which pass
radial tubes, one in each arm. From the radial tubes arise
lateral channels which communicate directly or through
bladder-like ampullae, with distensible feet which reach the
exterior by pores in the skeleton (Figs. 97, 98) . The tip of the
foot may be provided with a sucking-disc, serving as a means
of attachment and of locomotion. Frequently the walls of
these feet are thin and apparently serve for respiration, and the
terminal " foot " at the end of each radius may be highly
modified to form a sense organ (tentacle). The feet, the
ampullae, and even the radial vessels may be wanting. The
ring-canal, in typical forms, communicates with the surround-
ing sea-water by means of a tube (stone canal) which termi-
nates in a sieve-like plate, the madreporic body, through which
the water enters the water- vascular system. In the majority
of the Holothuroids the madreporic tubes open into the body

208

ZOOLOGY.

cavity instead of opening to the exterior. In consequence the
fluid which is found in the water-vascular system is the same
as that of the body cavity and contains amoeboid cells. In
the crinoids also the water-vascular system communicates
directly with the coslom, but there is no true madreporic canal.
In its stead is found a system of ciliated water-tubes in con-
nection with the ring canal. Identify the elements in the
water-vascular system from Fig. 98.

242. Respiration occurs in connection with the water-
vascular system especially in those forms in which the ten-
tacles and ambulacral feet are possessed of thin walls (holo-
thuroids and some echinoids). In the asteroids and echinoids

FIG. 97.

FIG. 97. Transverse section of. the arm of a Starfish near the disc. Diagrammatic.
Lettering as in preceding figure, a.r., ambulacral rafter (ossicle) ; ov., ovary, con-
taining ova.

Questions on Figs. 96 and 97. What are the principal sets of organs
represented in the disc of the starfish? Which of these have radial por-
tions going into the arms? Follow carefully the ectodermal, entodermal
and mesodermal boundaries. Locate and identify the various structures
lettered, and determine as far as possible, whether the essential part of
each is furnished by ectoderm, entoderm or mesoderm. Is there a coelom?
Your evidences? What is the relation of the water-vascular cavity to the
coelom, in origin?

ECHINODERMATA. 209

there are thin outpocketings of the body-wall, papulae or
branchiae (Fig. 97, d.b.), the cavity in which is continuous
with the body-cavity. The body fluids may thus be aerated
from the water outside. In some forms water is drawn into
special branching pockets (respiratory tree} in the wall of the
rectum, and later is forced out again.

243. Circulation. The circulatory vessels are merely
partly differentiated portions of the ccelom or body cavity.
Our knowledge is by no means complete but it seems that in
none of the groups is there a complete separation of the blood
spaces from the ccelom. There are probably no contractile
hearts. The walls of the blood spaces may bear cilia, which
assist in securing the motion of the fluid. The blood contains
migratory cells, usually colorless, and is identical with the fluid
in the body cavity. The general body contractions are im-
portant in causing motion of the. fluids. It should be remem-
bered that the water vascular system is also partly circulatory
in function. The blood vessels of the various classes agree
in having a central circular portion consisting of one or more
rings, with radial tubes running into the arms, and in some
instances vessels which accompany the intestine. The vessels
of the oral surface are, throughout, in close connection with
the nervous epithelium (Fig. 96, r.b.}.

244. Excretion, It is impossible to name any organs
known to be solely excretory in function. As in respiration
many organs may take part in the work. The gaseous and
soluble excreta are eliminated through the general body sur-
face, the papulae, the respiratory tree, or the ambulacral organs.
The skeletal ossicles in the mesoderm represent, in part at
least, the elimination of certain inorganic salts which can not
be used in the vital activities, and are therefore excretions.

'245. The Muscular System. The degree of the develop-
ment of the muscular system varies. In those forms which
have a well-developed skeleton the body muscles are not of
much significance. In the holothurians, on the contrary, the
15

2IO

ZOOLOGY.

-f

FIG. 98. Diagram of a portion of the water-vascular (ambulacral) system of the
Starfish, a, ampullae; /, ambulacral feet; m, madreporic body; p, Polian vesicles; r.c.,
ring canal, with the upper portion removed at the right of the figure; r.t., radial
water tubes (in r.t.' the upper portion is removed at the' distal end and the proximal
portion is represented entire) ; s, stone canal.

Questions on the figure. Where does the water enter this system
of vessels? At what points in the system is it of use? By comparing all
illustrations at your disposal, describe the mode of using this system of
organs for locomotion. How may it be used in respiration?

body is capable of definite and considerable contractions, by
reason of both circular and longitudinal fibres. In forms
with incomplete skeletons, as the star-fish, muscular fibres
connect the ossicles and there is a degree of flexion of the
arms. There are also special muscles controlling the water
vascular system, the stomach, the mouth parts, the spines and
pedicellariae. The fibres are non-striate.

246. The Nervous System consists of a ring around the
mouth and a radial nervous band in each arm supplying, by a

ECHINODERMATA. 211

plexus of fibres and cells, all the radial organs. This system is
superficial ("ventral ") to the radial water-tube (Fig. 97, r. n.}
and in the star-fish preserves its connection with the ectoderm
from which it is in all forms derived. Other deeper lying,
and even aboral, nervous elements are described for some of
the members of the group. These elements, when present,
have as their function the innervation of the muscles of the
interior and of the aboral wall of the body.

Sensory organs are not highly developed. The animals
show evidences of possessing a chemical sense (analogous to
taste and smell) by which the presence of food is detected.
This is apparently localized in the tentacles in such forms
as have them. A tactile sense is also present, and is most
highly developed in the tentacles, ambulacral feet, and other
movable outgrowths. At the tip of the antimeres of the
asteroids, or of the radial nerve (echinoid) are structures
bearing pigmented spots, which appear to be sensitive to light
(eye-spots}. These cannot give more than a very general
impression of light, by means of the chemical changes induced
in the pigment cells by the action of the light.

247. Reproduction is wholly sexual. The sexes are dis-
tinct, but the males and females are not often distinguishable
by external characters. The sexual organs, ovaries or testes,
are lobed bodies occurring usually in pairs in an interradial
position. These open by pores also interradial, and usually
dorsal (Fig. 97, r. p.). There are typically five pairs of genital
glands, but in the holothurians the number is reduced to one.
Fertilization takes place outside the body, and usually the
development is wholly independent of the parent. In some
instances however the parent has special pouches in which
development proceeds.

. 248. Development. The fertilized ovum undergoes total
and practically equal segmentation, producing a ciliated
blastula. Gastrulation occurs by invagination resulting in
ectoderm and entoderm. The mesoderm is formed in two

212 ZOOLOGY.

ways : ( i ) by migrating cells budded from the entoderm into
the segmentation cavity (mesenchyma; Fig. 12, c) ; and (2)
by the outgrowth of ccelomic vesicles or pouches from the wall
of the archenteron or entoderm (true mesoderm). These
latter outpockets of the wall of the gut are those which give
rise to the coelom and to the water vascular system (see

241).

In the later larval development the cilia of the gastrula
become limited to two zones, a preoral and a preanal,
and the shape of the larva is much modified. Numerous
paired, lateral outgrowths serve to accentuate the funda-
mental bilateral symmetry. In most members of the group
a marked metamorphosis occurs in the passage from the larval
to the adult condition. During this change, the water vascular
system and the mid-gut of the larva are retained with the
necessary modifications. About these as a centre, what we
might almost call a new animal, the radiate star-fish, begins
to grow at the expense of the larval organs which are ab-
sorbed by the amoeboid cells, and thus new organs appropriate
to the adult are formed. During this process the bilateral
symmetry of the embryo gives place to the radial symmetry
of the adult. While there is no reproduction by budding
there is a striking power of renewal of arms or other portions
which may be lost by injury, or in some instances by self-
mutilation. ^.rms are readily reproduced if the disc is un-
injured (stars, brittle-stars, and crinoids) ; portions of the
internal organs, as the digestive tract, are said to be regen-
erated by some of the holothurians. Occasionally, at least,

*

an arm seems to have the power of reproducing a new disc
and other arms. This power of throwing off arms and re-
placing them is doubtless a means of defense.

249. Ecology. The echinoderms are marine. The larvae
are free-swimming, pelagic, but after the assumption of
the adult form they usually become much less active. The
crinoids are typically stalked and often attached. The
asteroids and echinoids inhabit the bottom of the ocean where

ECHINODERMATA. 213

they creep more or less slowly. They may be found at almost
any depth, from the shallow pools at low tide to the deepest
bottoms. Many of them burrow in the mud and sand, and
others (some sea-urchins) have the power of scouring out
burrows in the rocks by the action of their spines. Echino-
derms, being slow movers, are compelled to subsist upon such
food as the currents or the chance movements of other ani-
mals may bring, or upon the debris which falls to the bottom
of the sea, or upon such organisms as are attached and cannot
escape. The star-fishes for example are a constant menace
to the oyster beds. The fact that some star-fish are in a meas-
ure gregarious makes this all the more true. It is difficult to
see how the star-fish can get the oyster from the protection of
its shell ; but it manages to get the shell open and clasping its
arms about its prey it turns the cardiac portion of its stomach
inside-out over the soft part of the oyster and thus leisurely
digests it outside its body, so to speak, leaving the empty shell
behind. Except for this the group .is of little economic im-
portance. The Chinese esteem some species of Holothuria
(the trepang, for example) as food. The group appeared
early in geological time and has had very characteristic rep-
resentatives in all ages up to the present. The changes which
have taken place in the echinoderms from one geological age
to another are among the most interesting and instructive
furnished by the invertebrates.

250. Classification. Class I., Blastoidea; Class II., Cystoidea.

(These are both extinct, fossil classes. They comprise stalked and
attached forms, and perhaps represent the nearest approach of our known
species to the primitive echinoderms.)

Class III. Crinoidea (feather-stars and sea-lilies}. These forms
are less common than in earlier geological times, when they must have
been very abundant and very beautiful. They contribute much to the
formation of the fimestone of the Palaeozoic. They are usually provided
with jointed stalks, by which they may be attached to the bottom. At the
summit of the stalk is a central disc with five arms often much branched
and bearing lateral pinnules. The anus is on the oral or upper surface,
the stalk arising from aboral surface. They are inhabitants of the deep
sea.

Class IV. Asteroidea (star-fishes) (Fig. 95). The asteroids, of which

214 ZOOLOGY.

there are several hundred species, are free echinoderms with a central
disc and usually five arms. The latter are large and contain liberal
coelomic spaces in which are lodged outgrowths of the digestive system
and other organs. There is a distinct oral and aboral surface. The anus
and madreporic body are on the latter. Distinct ambulacral grooves lie
on the oral surface of the arms. Adult star-fish may vary in size from
a few inches to two feet or more in diameter.

Class V. Ophiuroidea (brittle-stars}. These are fragile, free echino-
derms in which the arms are small and much more distinct from the
disc than in the asteroids. The organs of the disc are not continued
into the arms. There is no anus, no ambulacral grooves, and the madre-
poric body is on the oral surface. Their slender arms are useful in cling-
ing to supports or to prey, and are used in locomotion.

Class VI. Echinoidea (sea-urchins, sand-dollars). These are free
echinoderms without free arms. The arms are connected by the develop-
ment of interradial plates. The calcareous rods are united into plates
which produce a complete external skeleton varying from flat dome-shape
(as in sand-dollars) to a globular form (Echinus or Arbacia). The
mouth is usually in the centre of the oral surface and the anus near the
centre of the aboral, yet one or both may come to have an excentric posi-
tion. In this way the bilateral symmetry is accentuated at the expense of
the underlying radial symmetry. The madreporic body is aboral and there
are no ambulacral grooves. The spines of the urchins are usually well
developed and may be used to scour out rounded pockets in rock in which
the animals are sometimes found.

Class VII. Holothuroidea (sea-cucumbers'). These are soft, free
echinoderms, elongated, cylindrical or flat, with mouth and anus at
opposite poles of the horizontal long axis. The skeleton is not well-
developed, usually being represented merely by scattered spicules. The
water-vascular system in most forms communicates with the body cavity
instead of the exterior. Well-developed tentacles occur about the mouth.
Most holothurians burrow in the sand or mud, but others cling to rocks
near the surface of the water, and still others occur at great depths in
the ocean.

251. Suggestive Studies for the Library or Laboratory.

1. Read and report on the metamorphosis of the various
members of the group.

2. Study from dry and moist material and report on the
structure and mode of action of " Aristotle's lantern " in
Echinus.

3. Construct a table of parallel columns one for each of
the five living classes and contrast them as to: (i) general
form of body including symmetry, (2) manner of motion.

ECHINODERMATA. 215

(3) position of mouth and anus, (4) position of madreporic
body, (5) character of digestive tract, (6) differences in the
spines and other skeletal structures, (7) the position and
character of the ambulacral feet, (8) habitat and food, (9)
parts repeated in the antimeres.

4. Report on the habits, appearance, and abundance of
crinoids in geological time.

5. The origin and development of the water-vascular
system.

6. Compare the figures of the various classes as illustrated
in your reference texts and mark the degree of variation.

7. What evidences can you find for the statement that the
ancestors of the present Echinoderms may have been bilateral
forms ?

CHAPTER XV.
PHYLUM V. ANNULATA (SEGMENTED WORMS).

LABORATORY EXERCISES.

252. The Earthworm (Allolobophora or Lumbricus) .
The principal work should be done with living worms. For
whatever anatomical work is undertaken, specimens may be
killed by exposure to fumes of chloroform while wrapped in
cloth moistened with water; they should then be pinned out
straight, and hardened in an abundance of alcohol. If needed
in the winter they may often be found under manure heaps,
or about green-houses. They may be kept alive in flower pots
containing moist earth.

1. Promorphology; General Form. Is there an anterior
and a posterior end? How distinguished? Is there any dis-
tinction of dorsal and ventral surfaces? If so, what? Is there
bilateral symmetry? What external evidences of segmentation
do you find? How are the similar units (metameres or seg-
ments") arranged? Compare with the condition in the star-
fish. Compare the metameres of different parts of the body,
noting differences. Is the body divisible into regions (i. e.,
groups of similar metameres) ? Locate (by numbering the
segments) all such regions. How many segments in the ani-
mal? To what extent does this vary in different specimens?
Show by a series of diagrams the shape of the animal, and
the shape and size of cross sections in various regions.

2. Activities. Describe, after careful observation, the
method of locomotion in the earthworm. Place the worm on
a rough board: on a plate of glass. What is the difference?
And why? Compare the various parts of the body as to size,
during movement. Cause of the difference? Can each end
move foremost? What seems to determine which end shall
protrude as the result of the muscular contractions?

216

ANNULATA. 2 17

Does the animal respond equally to contact (with pencil or
toothpick) at anterior, posterior, and middle parts of the
body? Devise a method of determining whether it is sensitive
to light. Record results.

Place moist soil and dry soil side by side on a board ; place
the worm in various positions to test his preference. Record
results. Place a piece of filter paper which has been dipped
in acetic acid in the path of a worm. How does it react? Try
similarly a sugar solution; a salt solution; a decoction of de-
caying leaves. Will an earthworm pass into water? Do your
experiments bear in any way on the habits of the earthworm
in nature? Can you secure any evidence as to the food of the
earthworm ?

3. Special External Structures. Locate the mouth, the pre-
oral lobe, clitellum (a series of swollen segments), anus.
Compare preoral lobe with other segments. With a lens and
by drawing the worm backward between the fingers discover
the setae or bristles. Are they found on all segments ? Num-
ber and position of the groups of setae in each segment? What
is the function of the setse? Proofs?

4. Internal Anatomy. Pin out a worm, which has been hardened in
alcohol, on dissecting board or pan, and carefully remove the dorsal wall
from the anterior half of the body by making lateral incisions with sharp-
pointed scissors, or make a single incision along the back a little to one
side of the middle line. After noting the cross membranes {dissepiments),
their relation to the rings on the outside, and their attachments, cut them
so the body wall may be folded back and pinned. The dissection should
proceed under fluid, 50 per cent, alcohol, for example. Make all the out-
line drawings necessary to show all your discoveries. Notice the coelom.
Is it completely divided by the dissepiments? Are the chambers of equal
size?

(a) Digestive organs : Beginning at the anterior end, note trie following
regions :

Pharynx, a pear-shaped enlargement: how held in place? In what

segments is it situated?

(Esophagus, a narrow tube; crop; gizzard; intestine.
Determine the segments in which each region occurs. Does the
digestive tract show any signs of segmentation, *. e., in corre-
spondence with the external rings?

2l8 ZOOLOGY.

(&) Circulatory system : A living or newly-killed specimen is some-
what better for this. Discover, if possible :

Dorsal vessel (just dorsal to the digestive tract).

Ventral vessel (just ventral to the digestive tract).

Hearts, transverse vessels connecting the longitudinal vessels, in

segments VII to XL

There are other vessels more difficult to find. Examine a drop of
the contents of the blood vessels with the microscope.

(c) Reproductive System: These organs are rather too complicated
for satisfactory results in an elementary class. Instead of a detailed ex-
amination note the reproductive segments (in the region of the oesophagus)
with the whitish bodies showing at the sides of the alimentary canal, and
ventral to it. They are attached to the septa. (Compare figures in various
text-books.)

(d) Nervous System: In a well-hardened preparation, identify:
Brain, two whitish ganglia just dorsal to, and in front of the pharynx :
Collar, around the mouth, connecting the brain with ventral ganglia,

the first of a double longitudinal chain of ganglia which give off
nerves in each segment. How are the ganglia of the ventral chain
related to the dissepiments?

(tf) Excretory Organs : Just lateral to the nerve-chain the student may
be able to find coiled thread-like structures (nephridial tubes') in nearly all
the body segments (see text, 264). How many in each segment?

5. Microscopic Demonstration. The teacher should make or secure
good permanent mounts of transverse sections of the earthworm, by means
of which the students should make out the following points. (See Fig. 101.)
Cuticle, or outer layer.

Body-wall, and the relation of the circular and longitudinal muscles.
The ventral nerve-chain in position.
The dorsal and ventral blood vessels.
The wall of the digestive tract; gland cells, typhlosole, etc.

253. Dero (or other minute aquatic Annelid). Any one of
these fresh water worms may be used very profitably to sup-
plement the students' work on the earthworm. Mount the
living worm, being careful to support the cover-glass. Study
with low power. Compare at all points with the earthworm.
Dero may usually be had at any season of the year by taking
mud and organic matter from the bottoms of foul brooks or
ponds and placing it in vessels in the laboratory. The worms
will usually come to the sides of the vessels where they may
be seen. Owing to its transparent qualities, such a form will
be especially valuable in giving the student a better idea of

ANNULATA. 219

the performance of function in the group. What evidences
of muscular action are manifest? How is locomotion effected?
Position and mode of action of setae? Study the capture of
food; how is its progress through the digestive tract, and its
elimination therefrom effected? Do you discover any circu-
lation of the blood ? Direction of flow? Evidences? How ac-
complished? Test for ability to receive and respond to stimuli
of different sorts. Where are new segments formed? Dis-
cover, if possible, instances of fission, by which new individuals
are formed.

254. The Leech. The leech may be studied and compared with the
earthworm as to its external features, its habits, mode of locomotion, and
the like. If large specimens can be had some members of the class might
substitute it for the earthworm and the results of the studies brought into
comparison.

255. Nereis. If specimens of Nereis can be obtained this worm should
be compared with the earthworm. (Even two or three good specimens
may be made useful from year to year as demonstration both of external
and internal structure.)

Note especially:

(a) The specialization of the anterior end; proboscis, mouth, jaws,
palps, cirri, eyes, antennae.

(&) The fleshy supports of the bristles, parapodia.

DESCRIPTIVE TEXT.

256. The Annulata are separated from the unsegmented
worms by the possession of a series of segments or metameres
which show on the exterior as rings, and contain similar or
homologous organs or similar portions of a continuous organ.
There is also a more uniform development of the ccelom than
in the lower worms. They differ from the ccelenterata and
echinoderms in having bilateral rather than radial symmetry in
the adult condition. The development is often direct, but in
many, especially the marine forms, there is a metamorphosis.
The larva has a peculiar balloon-shaped form, known as the
trochosphere (Fig. 104, E), similar in some respects to the
Rotifers.

257. General Characters.

i. Body elongated, bilaterally symmetrical and segmented.

22O

ZOOLOGY.

FIG. 99.

2. External paired appendages (setae, bristles, etc.) not
jointed.

3. There is usually a well-developed body cavity.

4. The excretory organs are typic-
ally paired nephridial tubules, one pair
in each segment, connecting the body
cavity with the outside. Certain
highly modified pairs of these serve
as outlets for the reproductive bodies.

5. The nervous system consists of
(i) a supra-cesophageal ganglion
(brain), and (2) a circum-cesopha-
geal collar or connective uniting it
with (3) a ventral chain of ganglia
with a ganglion in each segment.

6. Locomotion is primarily effected
by means of the contractions of the
body wall, acting on body fluids in the
cavity within.

FIG. 99. Dero, a fresh-water oligochaetous annelid,
in optical (frontal) section. Enlarged 30 times, a,
appendages; br., brain; d, dissepiments; i, intestine;
m, mouth; nph, nephridium; oe, oesophagus; p, pavil-
ion, lined with ciliated entoderm; ph., pharynx; pr.,
processes from the anal segment; s, zone immediately
in front of the anal segment where new segments are
continually being formed; z", the zone of fission or
budding. This takes place in the middle of a seg-
ment. The anterior half-segment of s' will produce
a region like s for the anterior half of the worm.
The posterior half -segment will produce a head and
four segments like those which contain the pharynx
(1-4) of the parent worm.

Questions on the figure. What regions
of the digestive tract are sufficiently differ-
entiated to deserve notice? What is the
number of the segment in which fission is
taking place? What structures must the
anterior half of this segment make? The
segment behind the dividing segment becomes
"-/>. "*"* number 5 of the new posterior worm. What
structures then must be developed from the
posterior half of the dividing segment?

ANNULATA.

221

7. Development may be either direct or indirect.

258. General Survey. The Annulata though conforming
to the type outlined above are very diverse in appearance,
habits and internal structure. While the Chaetopoda, the
class to which the forms studied in the laboratory belong,
are taken as the type, the leeches, which have no bristles but
possess suckers, are undoubtedly related, as is shown by their
development. The Rotifers and other forms are sometimes
included among the relatives of the Annulata. Metamerism
in animals is a most interesting phenomenon to zoologists.
This group is the first in which we have found true metamerism.
The body of the animals is more or less constricted on the

d. m.

n. c.

n. f.

CO.

FIG. 100. Longitudinal section of anterior end of Dero. A, sagittal section; B,
frontal section to show anterior portion of nervous system, b, brain; co., nervous
collar about the mouth; c.v., contractile blood vessels ("hearts"); d, dissepiment;
d.m., dermo-muscular wall; d.v., dorsal blood vessel; m, mouth; n.c., nerve cells; n.f.,
nerve fibres; np., nephridia; p, prostomium; ph., pharynx; s, setae; sn., segmental nerves;
v.g, ventral chain of ganglia; v.v., ventral blood vessel. Only a portion of the blood
vaseular system is shown, and this appears unsectioned in the figure.

Questions on the figure. Compare this with the cross-section of Dero
and identify the parts. How do the four anterior segments differ from
the others figured? Does the ventral nerve cord continue the whole length
of such an animal as this? Which organs may be described as segmental
and which as continuous through the segments?

222 ZOOLOGY.

outside into rings as the name (Annulata) implies. The
internal organs also show metamerism, but in various ways.
These organs may pass directly through, with slight segmental
modification, as the digestive tube and ventral nerve cord;
they may be repeated independently in each segment, as the
setae or nephridial tubules ; or they may be represented in only
one or a limited number of segments, as the brain or the
reproductive bodies. The segments are not therefore exactly
equivalent, yet the agreement between successive segments is
sufficient to merit the term homonomous (see 121). The
number of segments varies from three to hundreds. The body
is from four or five to many times as long as broad, and is
usually cylindrical or flattened dorso-ventrally.

259. The dermo-muscular sac is composed of the integu-
ment or skin and the muscular layers of the body wall. Being
filled with the body fluids it is a very important instrument
of locomotion. This is accomplished by the alternate con-
tractions of the circular and longitudinal fibres with which
the wall is supplied. Externally there is a cuticula, usually
very thin, overlying and secreted by the layer of epidermal
cells. Some of the cells of the epidermal layer are glandular
and others are sensory. The setae or bristles are secretions of
the epidermal cells and lie in sacs in the skin. These struc-
tures vary in number and position but are usually paired,
two or four groups to each segment. They are absent in the
leeches. Next to the skin is a layer of circular muscle fibres,
and within these are the longitudinal bands of muscle fibres.
In the leeches there are also dorso-ventral fibres. Special
groups of fibres occur in connection with the setae, the mouth
parts, suckers, etc. The fibres in worms are spindle-shaped
and unstriate. The dermo-muscular wall bounds a true body
cavity in the chaetopods; but in leeches the coelom is almost
filled with connective tissue. This suggests the condition in
many of the unsegmented worms. See Figs. 99, 101.

223

c

J

FIG. 101. Transverse section of Dero. x 300. c., coelom; c./., cells of the so-called
"lateral line"; d.m., dermo-muscular wall including muscles and skin; d.v., dorsal
blood vessel; ect, ectoderm; ent, entoderm; g, gut; g.f., giant nerve fibres; gl, glandular
cells assisting in digestion; m.c., circular muscle fibres; m.l., longitudinal muscle fibres;
n, nephridium; n.v., ventral nerve chain, made up of nerve cells and nerve fibres; s,
setae; i\v. t ventral blood vessel.

Questions on the figure. Compare this with Fig. 100 and identify
all the structures which appear in both. What elements enter into the
dermo-muscular wall ? Identify nerve cells, fibres and the " giant fibres "
in the ventral nerve cord.

260. Worms as a rule have no external skeleton other than
the cuticle, but in some instances a tubular protective structure
is formed by secretion or by cementing together small particles
of foreign matter. Because of the absence of hard skeletal
parts little is known concerning the worms of past geological
ages.

261. Digestive System. The stomodaeum, the mesenteron,
arid proctodseum (see 90) are all to be distinguished in the
digestive canal. The mouth is not quite terminal, but slightly
ventral. The prostomium (or preoral lobe), a muscular ex-
tension of the oral segment, overarches it. There is typically
an enlarged muscular pharynx which is often eversible, fol-
lowed by a narrow tubular .oesophagus. Often there is no

224 ZOOLOGY.

further differentiation, the remainder of the tube being fairly
uniform and called the intestine. Frequently however special
enlargements occur, chief among which is the stomach. In
the leeches the alimentary system is much modified in accord-
ance with the blood-sucking habit of the animal. The crop
is capable of great enlargement and may contain enough
blood to nourish the animal for a long time. The mouth is
sometimes armed with special cuticular outgrowths which
serve as teeth. Glands either unicellular or compound occur
in various regions of the digestive tract. In the earthworm

FIG. 102.

ent.

FIG. 102. Transverse section of the intestine of the Earthworm, ty, typhlosole, an
infolded longitudinal ridge in the gut in which the gland cells (.gl.) are especially
aggregated. Other letters as in Fig. 101.

Questions on the figure. Of what conceivable gain is the typhlosole?
What is it analogous to in the higher types of animals?

and related forms there is a dorsal longitudinal fold of the
intestinal wall into the lumen of the tube, thus increasing the
exposed surface. This is called the typhlosole (Fig. 102) and
is supplied with cells which have been described as digestive.
The entodermal epithelium may secrete a cuticle or may be
ciliated. This layer is surrounded by a connective tissue and
muscular fibres.

262. Respiration is effected for the most part through the
general body wall, into which the blood capillaries or the

ANNULATA. 225

lacunae of the coelom may penetrate. In some forms there
are special thin places and outpocketings of the body-wall
(branchiae) by which the exchange of gases is facilitated.
These are characteristic of the Polychaeta especially (Figs.
105, 106).

263. Circulation. In some of the simplest worms there
are no special blood vessels. The coelomic spaces contain a
fluid, which possesses corpuscles and is moved by the general
body contractions. In the typical condition there are two or
more longitudinal vessels, dorsal and ventral (or lateral) in
position. These are often connected by transverse loops in a
few or many segments of the body especially at the anterior
and posterior ends. The circum-intestinal loops are often
contractile, and the longitudinal vessels may show a wave of
contraction passing from one end to the other. Capillaries
vary much in perfection of development.

264. Excretion takes place by means of the segment al
organs or nephridia, of which there is usually one pair in each
segment, with the exception of some of the anterior segments.
The nephridium is a tubular structure consisting essentially of
the following portions (Fig. 33) : (i) a ciliated funnel, com-
municating with the coelom; (2) a tortuous glandular region;
and (3) an outlet through the body wall, often supplied with
muscle fibres. The nitrogenous waste products find their way
into the fluid of the coelom and thence into the nephridium,
or directly into the nephridium from blood capillaries which
may occur in its walls, and thus are finally eliminated upon
the exterior of the body.

265. Nervous System. The " central " nervous system
may be said to consist of three portions : ( i) a mid-ventral line
of nerve fibres, and nerve cells which are diffusely scattered
or collected in ganglia, (2) a brain which is anterior and
dorsal to the pharynx, (3) a connective or collar about the
pharynx connecting (i) and (2) (Fig. 100). The brain and

16

226 ZOOLOGY.

ventral cord may show distinct right and left lobes or may
be completely fused into a median mass. From the brain,
nerves pass to the head-parts. From each of the segmental
portions of the ventral chain nerves pass to the walls, viscera,
etc. The ventral cord frequently lies in a blood sinus which
secures its abundant nourishment (leeches).

The sense organs occur very unequally in the group. The
Polychaeta and the leeches are best supplied. The skin is
generally sensitive to contact and chemical stimuli. This sensi-
tiveness is perhaps specially localized in the tentacles, cirri,
and more movable parts. Otocysts, fluid-filled cavities
bounded by sensory epithelium, occasionally occur (see 108).
Some solid particles (otoliths) float in the fluid. These have
been described as organs of hearing but the sensation resulting
is probably quite different from what we know as hearing.
They are apparently organs of equilibration, enabling the
animal to appreciate its position in relation to the pull of
gravity. Eyes may consist merely of a group of pigmented
cells with nervous connections, or may be very complicated,
consisting of a capsule with refractive media and retina.
Images of objects are not formed, in all probability, but the
direction and intensity of light can be appreciated. In the
leech there are sense organs in each segment somewhat similar
in structure to the eyes. Their function is unknown.

266. Reproductive Organs. The Oligochseta and the
leeches are hermaphrodite. In the Polychseta the sexes are
separate. The sexual products are developed from the ccelomic
epithelium, sometimes on the dissepiments, sometimes on the
body wall, or in other special regions. The elements may be
produced in many segments (Polychaeta), or in a few anterior
ones (Oligochseta). The region is usually distinguishable
only about the breeding time. In the hermaphrodite forms
the ova' and spermatozoa often mature at different times and
are produced in different segments. This of course insures
cross-fertilization. In the Polychaeta the conditions are rela-
tively simple. The elements are freed in the body cavity and

ANNULATA.

227

when mature find their way into the water where fertilization
takes place. The organs are much more complicated in the
hermaphrodite worms. The spermatozoa are produced in
the testes, are passed into the seminal vesicles where they
are matured, and at the time of copulation escape to the
exterior by the vasa deferentia, to be deposited in the
sperm sacs or rcccptacula seminis of another worm. From
this place, any time after copulation, the sperm is brought into
contact with the ova as they pass from the ovary, where they
are produced, to the egg-sac or to the exterior; or the sperm
and ova may be brought together after both have escaped from
the body. It is believed that in some instances at least the
genital ducts are modified nephridia.

267. Reproduction and Development. Sexual repro-
duction is universal. As we have seen, copulation may occur
or the elements may come together in the water. In the
Oligochseta and leeches the fertilized ova, or the ova together
with masses of spermatozoa, are enclosed in a cocoon of
secreted material and within this case the young worm is de-
veloped. In the Polychseta the larva undergoes its develop-

FIG. 103. Two stages in the development of Nereis. A, 8-celled stage; B, i6-celled
stage, both viewed from the active or ectodermal pole, mi. 1 , mi. 2 , and mi.*, the first,
second and third sets of micromeres separated from ma., the macromeres; s 1 , first
somatoblast, one of the second group of four cells to be budded from the macromeres;
s 2 , second somatoblast, one of the third group, which gives rise to the mesoderm. The
micromeres are ectodermal and the macromeres produce the entoderm. (After West-
inghausen.)

228

ZOOLOGY.

FIG. 104. Diagrams of stages in the metamorphosis of Polygordius, a primitive
annelid. Ectoderm throughout is represented as nucleated without cell boundaries; the
entoderm has the cell-boundaries shown, and the mesoderm is diagonally shaded. A,
gastrula; B, same with blastopore closed; C and D represent formation of stomodaeum
and proctodaeum from ectoderm; E, Trochosphere stage showing formation of segments
in the posterior portion; F, adult (sagittal); G, adult (transverse), a, archenteron;
bp., blastopore; br, brain; c, ccelom; d, dorsal; di, dissepiments; m, mesenteron; pr.,
proctodaeum; s.c., segmentation cavity; st, stomodaeum; v.n., ventral nerve chain; s,
zone of formation of nerve segments. After Fraipont.

Questions on the figure. Trace the behavior of ectoderm and ento-
derm in these figures and determine what structures each seems to give
rise to. What is a Trochosphere? Distinguish between somatic (body)
and splanchnic mesoderm. (See 56.)

ment in a free state. Segmentation in Annulata is complete
and usually unequal, giving rise at the eight-celled stage to
four micromeres and four macromeres (Fig. 103). The
micromeres produce the ectoderm; directly or indirectly the
macromeres give rise to the entoderm. Early in the cleavage
" primitive mesoblasts " cells which produce the mesodermal
structures, are separated from the macromeres. A gastrula

ANNULATA. 229

is formed either by invagination or by overgrowth. In the
earthworm (Oligochseta) the blastopore of the gastrula forms
the mouth of the adult worm. In Nereis ( Poly chaeta) the
blastopore closes by growth, and the stomodaeum and proc-
todseum arise by ectodermic invaginations which finally be-
come continuous with the entoderm of the archenteron (Fig.
104, D, of Polygordius}. A ciliated, free-swimming larval
stage ensues, known as a trochosphere (Fig. 104, ). The
trochosphere may be looked upon as representing the anterior
or head end of the adult. The later metamorphosis to the adult
condition involves the reduction in size of the enormous
anterior region, and the growth of segments at the posterior
end, and is characteristic of Polychaeta. The development of
leeches is direct as in the Oligochseta, or in some instances it
might be more accurate to say that the process of metamor-
phosis is very much abbreviated, being completed by the time
of hatching.

268. In addition to sexual reproduction many worms, par-
ticularly the aquatic forms, have the power of multiplying
by fission. In some instances this may consist of a mere
breaking in two, as was seen to be possible in the star-fish,
each part regenerating segments corresponding to those lost.
In other cases (Nais, Dero, etc.) zones of rapidly forming
segments are produced somewhere in the mid-region of the
body, and from this zone a new head is developed for the
posterior zooid and a new tail for the anterior zooid, which
usually become structurally complete before the separation
takes place (Fig. 99, z'}.

In some of the Polychaeta (as Autolytus) a distinct alter-
nation of generation is found in which sexual and non-sexual
individuals are of very different appearance.

When artificially mutilated the earthworm, and some other
types as well, may regenerate the lost portions. Groups of
segments of one worm may be grafted upon another, complete
healing taking place in such a way as to produce an apparently
normal worm. Pieces may be grafted on the side of another

230 ZOOLOGY.

worm in such a way as to produce a forked or otherwise
abnormal result.

269. Ecology. The leeches are aquatic in habit and
many of them live on the blood of higher animals, a kind
of temporary parasitism; the Polychaeta are marine, and the
Oligochseta are chiefly fresh-water or terrestrial in habit. A
few of the latter groups are parasitic. Of the aquatic worms
some are actively free-swimming, others crawl in and out
among the living and dead matter of the bottom, others bur-
row in the sand, or secrete a tubular skeleton into which they
may retire. Their chief economic importance is that they serve
as food for fish and other food-animals. The earthworm, in
forming its underground burrows, eats its way into the earth,
swallowing the soil for the organic matter which it contains
and passing it through its digestive tract. These castings
may often be seen at the mouth of the burrows. Worms thus
break up the soil, making it more porous and accessible to
moisture, bacteria, and the rootlets of plants. Darwin esti-
mates that three inches of the subsoil is thus brought to the
surface in fifteen years through this agency.

270. Classification.

Class I. Chatopoda (bristle-footed}. Annulata with metameres usu-
ally well-marked both externally and internally ; with setae developed from
the epidermis. The ccelom is usually voluminous and is divided into
chambers by transverse dissepiments. Closed blood-vascular system.
Ventral nerve-chain ordinarily with a distinct ganglion to each segment.

Sub-class I. Polychceta {with numerous bristles). Marine Chsetopoda
with numerous setae typically borne on elevations of the body wall (para-
podia). Head usually well differentiated, bearing eyes, antennas, cirri, etc.
Branchiae or gills often present. Sexes separate; the reproductive organs
simple, and repeated in many segments. A metamorphosis occurs ; the
larva is known as a trochosphere.

Nereis, the " sand worm " of fishermen is a type of this group.
Autolytus is a small worm especially interesting because of its power of
reproducing by fission. The bud which is freed from the hinder end of
the worm differs from the parent stock in that it is sexual. Amphitrite
is a beautiful worm which represents the attached or tube-forming types.
As the result of their habits such forms tend to lose their segmentation
and the appendages of the posterior part of the body. The gills and
tentacles accumulate about the head. These and other types grow abund-

ANNULATA. 231

antly in the sand and mud of harbors, amid the vegetation of the bottom,
and over exposed objects of all sorts from low water mark to unknown
depths. Their value in utilizing debris and the more minute organisms
as food and thus becoming a link in the saving of these to serve as food

FIG. 105.

FIG. 105. Amphitrite ornata, from Vert-ill's " Invertebrate Animals of
Vineyard Sound."

for the higher organisms cannot be over-estimated. (Figures 105 and 106.)
Subclass II. Oligochcsta {with few bristles). These are Chaetopoda
with no parapodia and comparatively few setae which usually occur in
two or four clusters in each segment. They are mostly fresh water or
terrestrial in habit. Fleshy outgrowths, such as gills, are almost uni-
versally absent. The sexes are united in one individual and the accessory
reproductive organs are very complicated. Ovaries and testes limited
to a small number of anterior segments ; development direct. The head
riot so highly specialized as in the Polychseta.

The earthworms, of which there are numerous species, are the best
known types of this subclass. The genera and species are distinguished
chiefly by the position of the sexual organs. The aquatic Oligochaeta,
which are much smaller, are found in practically all ponds and ditches
where organic, matter is decaying. The more common genera are Dero
(Fig. 99), a beautiful, almost transparent worm which often forms a tem-
porary tube for itself of particles cemented by its own secretion, and
Tubifex, a longer worm which burrows in the mud at the bottom of
streams ; a portion of the body protrudes from the mud and waves gently

232

ZOOLOGY.

back and forth in the water. They may occur so thickly that thousands
may be seen in the space of a few feet. When their home is jarred they
speedily withdraw from sight. A colony of Tubifex nearly always has
associated with it one or more genera of smaller worms, as Dero or Nais,
a species similar to Dero but with eye-spots. Dero has an interesting
respiratory apparatus at the posterior part of the body (Fig. 99, />.), one
of the few instances where Oligochaeta possess such organs.

FIG. 1 06.

FIG. 1 06. Cirratulus grandis, from Vet rill.

Questions on Figs. 105 and 106. Are these Chaetopods? What are
your evidences? What is the -nature and function of the numerous out-
growths (branchial cirri) ? In what respects are they differently arranged
in the two types? Are these Oligochaeta or Polychaeta? Your reasons?

Class II. Discophora (bearing suckers}. Annulata in which there are
secondary external rings which tend to obscure the metameres, inasmuch
as the external and internal segmentation do not coincide. There are no
bristles. The body cavity is much reduced by the growth of muscles
and connective tissue. The remaining spaces contain blood and are in
communication with the vascular system. Two sucking discs are present
and are powerful organs of attachment. The anterior sucker embraces
the mouth ; the posterior is near the anus. Sexes are united in one indi-
vidual ; testes numerous, ovaries a single pair. Development direct.
Marine, fresh water, terrestrial, or parasitic in habit.

ANNULATA. 233

271. There are several other groups of annulates of considerable inter-
est to the zoologist which it seems necessary to pass by with mere
mention.

Class: Archi-annelida ; a few primitive forms, as Polygordius (Fig.
104).

Class: Sipunculoidea (Gephyrea'}. With traces of segmentation in
the embryo, but not in the adult.

Class: Chsetognatha (arrow worms).

Some authors would place here also the Rotifers (see 230).

272. Suggestive Studies for Library and Laboratory.

1. Look up the characteristics of the Archi-annelida, the Gephyrea, or
Sagitta, and report on their likenesses to the types studied.

2. On what grounds might the rotifers be associated with the annulata?

3. Compare the " segments " in cestodes and annulata.

4. In the Chsetopoda which sets of organs pass through all the seg-
ments, which are repeated in essentially all, and which are limited to a
few?

5. Examine and report on the habits of the earthworm. (Study in its
natural haunts or in box of moist earth in laboratory.) What are its
haunts? Method and rate of burrowing? Does it avoid water? What is
its food? How taken? Does the animal prefer light or darkness?

6. If near the sea-shore select other forms and report in a similar way.

7. Investigate parasitism among the Annulata.

8. What is the economic value of the earthworm? Of other worms?

9. Make a study from the text-books of the reproductive organs in any
of the hermaphrodite Oligochaeta.

10. In how many species of aquatic Oligochseta do you find reproduc-
tion by fission? In what particulars does the process seem to differ in
the different species?

11. Outline the life-history of Autolytus, including the origin of the
sexes.

CHAPTER XVI.

PHYLUM VI. MOLLUSCA.

LABORATORY EXERCISES.

273. The Clam (Mya) or Mussel (Anodonta, Unio).
Either the marine or the fresh-water type will serve. The
latter are to be found in almost all our streams and small
lakes. They may be obtained with a long handled rake from
the shore or from a boat. They often occur partly buried in
the sand or mud. If kept in water they may be transported
to the laboratory and placed in a tub of water with a few
inches of sand at the bottom, where something of the physi-
ology may be studied with profit. If they cannot be collected
when needed for study, care should be taken to supply plenty
of the preservative fluid in which they are kept.

i. The Living Animal. What facts were observed, in col-
lecting the material, concerning their haunts, their abundance
in different localities, their range in size, etc. ? Are there any
efforts at active feeding, as far as you have seen ? Do all your
specimens belong to the same species?

From the specimens in the tub make out the following
points :.,

Has the animal power of voluntary motion ? If so, what of
its rate, manner, the position of the animal during motion?
How is the animal supported in this position? Determine an-
terior and posterior ends, right and left sides, dorsal and
ventral surfaces. To what extent can the soft parts protrude
from the shell ? Note briefly, for later reference, the position
of all visible structures. How widely does the shell open
during life? Note the trail. With a pipette place a drop of
some colored but harmless fluid (carmine in water) near the
fringes of the posterior end, and note the results. Vary by
introducing salt, sugar, and acid in the solution. Devise

234

MOLLUSCA. 235

experiments to test whether the animal shows sensitiveness
to stimuli of various sorts : jars, contacts, currents in the
water, light, warmth and cold.

2. General Form. Renew your observations concerning
the symmetry of the clam by careful examination of the ani-
mal. Determine and show in a sketch all the points distin-
guishing the anterior and posterior ends. Are the right and
left halves symmetrical? Use a pair of empty shells for com-
pleter study.

The shell : what is the relation between the valves ? How
are they held together? Are they normally open or closed?
Give your evidences? To what extent may the shell open
without violence? How does the shell vary in thickness at
various parts? Contrast the interior and exterior as to finish
and markings. Make note of everything found, with outline
drawings, showing position. Locate the following regions
and structures : hinge, umbo or beak (a prominence near the
hinge), hinge ligament, hinge teeth, pallial line (a slight de-
pression marking the attachment of the mantle muscle),
muscle impressions, lines of growth. Review after studying
soft parts.

What is the oldest portion of the shell? Evidence. How
does the shell grow? How did the internal depressions come
to be? Evidence.

What layers are discoverable in a broken shell? How do
the inner, outer, and middle layers differ in thickness and

appearance

Do you find any differences worthy of note in different
individuals ?

3. Soft Parts. Remove one valve (say the left) by cutting
the two muscles which hold the valves together. Leave all
the soft parts in the right valve as little disturbed as possible.
Make a sketch showing the relation of the body to the shell. If
there is any difficulty in cutting the muscles, the clam may be
made to open by immersing it in water heated to about 140 F.
Identifv:

236 ZOOLOGY.

Left mantle flap. How related to the right? to the shell?

Siphons : modifications of the posterior margins of the
mantle. (These will be conspicuous or rudimentary in ac-
cordance with the species studied.) Number?

Adductor muscles of the valves ; number and position.

Mantle cavity. Separate the right and left mantle lobes
along the ventral margin, except in the region of the siphon,
and fold back the left. Where is it attached to the body?
The ventral or incurrent siphon opens into the branchial
chamber, the dorsal or excurrent into a smaller dorsal cham-
ber, the do acal. Verify and sketch.

Gill plates or sheets; number and attachment. Are they
symmetrical on the two sides? The eggs and developing em-
bryos may be found in the outer gill cavity at favorable times.
(A special study and report may be profitably made by some
student on the structure of the gills as shown by a hand lens
and the low power of the microscope. A bit of the living
gill from a fresh specimen should be examined.)

Abdomen, the soft, fleshy mass between the pairs of gills,
which terminates in a more solid part,

Foot : position and form ?

Mouth and labial palps ; at the anterior end and just below
the adductor muscle. How many palps?

(It is to be remembered that all the structures examined
thus far are external organs. The body wall has not been
penetrated at all. If it is the plan to study the anatomy more
closely, the following are the chief sets of organs deserving
attention. )

4. Other systems of organs.

Circulatory system. Open the pericardial cavity, just beneath the hinge
and a little posterior thereto, find the

Heart: auricles and ventricle. In a fresh preparation the contrac-
tions of the heart may be observed.
Vessels : one passes in each direction, but they are not easily seen

without injecting.

The intestine passes through the ventricle without open communica-
tion with it.

MOLLUSCA. 237

Excretory organ.

Organ of Bojamis, or kidney, lies just beneath the floor of the

pericardial cavity, one part on either side.

Each portion is a dark-colored sac, with an abundant blood supply.
Nervous system. (Traced best in hardened preparations.)
Visceral ganglia. Look between the gills in the posterior portion of
the body, beneath the posterior adductor muscle, and in the floor
of the cloacal cavity. Number, and closeness of connection? By
careful dissection determine what nerves leave them. Trace a
pair of these forward to the

Cerebral ganglia, on either side the mouth. Note the connections
between the cerebral ganglia. Trace from these ganglia the con-
nectives which pass ventrally to the
Pedal ganglia in the muscular foot, close to its union with the

abdomen.
Make a clear diagram showing the relations of these three pairs of

ganglia.

Digestive system.

Begin with the intestine at the heart. Trace posteriorly to the anus.
What is its relation to the posterior adductor muscle? Pass a
bristle into the intestine anteriorly and use it to guide the dissec-
tion. Trace the intestine through the abdominal mass, and plot its
course. Identify the stomach, the oesophagus, and the mouth. The
liver is a brownish or greenish mass surrounding the stomach.
Much of the visceral mass through which the intestine coils is made
up of the large reproductive glands which- open into the mantle
cavity.

5. Cross Sections. A series of cross-sections may be made by the
teacher, numbered, and used with profit as demonstrations. For such
sections the soft parts of the animal should be hardened for 24 hours in
I per cent, chromic acid; then one day each in 70 per cent, and 90 per cent,
alcohol. Keep in 95 per cent, alcohol for a few weeks. Cut one fourth
to one third inch thick and number so as to' be able to locate position
of section. Float in dish of alcohol and identify the parts found. Make
sketches of sections passing (i) through the stomach, (2) through the
heart, and (3) through the middle of the posterior adductor muscle. In
the absence of these the student should be encouraged to make a diagram
of an imaginary cross-section through the middle of the body. Include
the shell.

274. The Oyster. One or two students should be asked to prepare a
report on the structure of the oyster and present to the class an account
of the chief points of contrast between the oyster and the clam. The adult
oyster is fixed by one of its valves. Is it the same one in all specimens?

275. The Pond Snail (Limnaa).

i. The Living Animal. Observe, both in its natural home
and in glass vessel containing water in the laboratory.

238 ZOOLOGY.

To what does the animal adhere in the water ? Must it have
solid support? Can it swim? What is its method of locomo-
tion? What does it eat, and how? Can you determine whether
it uses the air in breathing or gets its oxygen from the water?
Proof? How is the gliding motion effected? Watch, with -a
lens, one crawling along the side of the glass vessel. Record
signs of sensitiveness to stimuli, by experiments of your own
devising.

2.. General Form. Is there any sign of bilateral symmetry?
In what parts? How are anterior and posterior distinguished?
Relation of the shell to the animal ? Identify :

Head : tentacles, number and position ; eyes, number and
position.

Foot, the muscular expansion : shape, changes in form and
position.

Mouth.

Respiratory orifice, position. Under what circumstances
seen?

3. Shell (secure empty ones). Make sketches of the shell
and identify the structures referred to in the following terms:
apex, aperture, lip, spire, whorl, suture, columella. (See
Fig. 1 08).

How would you describe the direction of the spiral? How
many whorls ? Have the young and old the same number ? Can
you detect lines of growth?

4. Soft Parts. These may be removed by dropping the animal sud-
denly into hot water, and then gradually twisting the soft portion from
the shell. It will scarcely repay the trouble to do more than re-identify
the following parts : mouth, respiratory orifice, mantle and mantle chamber,
and collar (a portion of the mantle). The spiral is occupied by the diges-
tive tract, its glands, the reproductive bodies, etc.

5. Development. Examine the stems of plants and the sides of the
vessel in which snails have been kept for some days for gelatinous cap-
sules of eggs. They are almost transparent and the eggs may be easily
located. What seems to be the value of the gelatine? Number and ar-
rangement of the eggs? What is the shape of the eggs? Get the earliest
stages possible, and watch day by day at short intervals, or compare cap-
sules of different ages. If care is taken, some idea of the early seg-
mentation stages may be obtained. Look for the blastula : are the cells of

MOLLUSCA. 239

the same size? Do you find a gastrula? What are the first signs you find
of differentiation of parts? Look for different stages of the later develop-
ment. It will not be profitable to try to follow the changes in detail.

276. A very valuable laboratory exercise may be had by comparing
large numbers of shells of a single species, found under varying condi-
tions. Compare as to shape, markings, etc., and see whether there are
individuals connecting your extreme groups. The land snail {Helix, Fig.
119) is favorable for such study.

277. The -Squid. The teacher should at least have a few specimens of
the Squid, from which the pupils may be required to get some idea of the
general form. Drawings should be made, showing all external features.

Note particularly :
Head : tentacles, number, comparative length ; suckers on the inner

surface, arrangement of suckers.
Eyes : number, size, position.

Olfactory organs opening beneath folds of skin behind the eyes.
Neck.

Body : general shape. It is surrounded by the
Mantle ; note the fin expansions at the posterior end. Where are

the attachments of the mantle to the body?
Siphon; how related to the mantle cavity?

What are your conclusions as to the symmetry and the normal position
of the squid ? Do you find anything from your external examination which
would lead you to class it with the clam and the snail?

DESCRIPTIVE TEXT.

278. The group Mollusca embraces from 10,000 to 20,-
ooo living species among which there are very great differ-
ences, as illustrated by forms as unlike as slugs, snails, oysters,
clams, devil-fishes, and squids. With the exception of a few
they are sluggish animals, and largely aquatic or frequenters
of moist places. Some are well protected by external armor
and others are perfectly naked. The typical adult mollusk
is clearly marked off from both the radiate animals such as
echinoderms and the segmented animals such as the Arthro-
pods and the Annulata, but some of the simpler types of
mollusks, and the larvae of certain of them which undergo a
metamorphosis, strongly suggest that they may be related to
some of the unsegmented worms.

279. General Characters.

i. Body soft, unsegmented, bilaterally symmetrical and
without segmented appendages.

240

ZOOLOGY.

2. The organ of locomotion is a muscular thickening of the
body, called the foot, which is variously modified.

3. A thickened dorsal fold of the body wall, called the
mantle, is usually present. This encloses a space, external to
the body, known as the respiratory chamber.

4. The mantle secretes in many cases a calcareous shell, -at
first single and symmetrical, but usually becoming either spiral
or separated into a right and left valve.

5. The central nervous system usually consists of three sets
of ganglia: (i) the cerebral or " brain," above the mouth, (2)
the pedal, in the foot and connected with the cerebral by
nerves, and (3) the visceral, also connected with the brain
by a pair of nerves (Fig. 36).

6. Except in the headless forms (Acalephs} a tooth-bear-
ing ribbon, the odontophore, is found in the mouth.

-atf.jb.

FIG. 107. Shell of a Bivalve Mollusk, inner surface, ad. a., depression showing the
attachment of the anterior adductor muscle; ad.p., posterior adductor muscle; h, hinge
with teeth; /, attachments of the ligaments; p, pallial line, marking the attachment of
the mantle muscles; s, the pallial sinus, marking the attachment of the retractor
muscles of the siphon; , umbo or beak.

Questions on the figure. Which is the dorsal and which the ventral
portion of the shell? Is this the right or left valve? What is the effect
of the contraction of the adductor muscles? What is the value of the
teeth on the hinge? To what point in the shell of the snail does the
umbo correspond?

MOLLUSCA.

241

280. General Survey. The more commonly known forms
are easily recognizable by the hard calcareous shell which
protects the soft unsegmented body within. The shell may be
in two sub-equal valves, right and left, or may be in one piece,
in which case it is usually coiled or spiral (Fig. 108). The
bivalved types are able to open and close the shell after the
manner of a box, and the soft parts are further capable of

FIG. 1 08.

c

FIG. 108. Helix. A, an empty shell in section from apex to base, a, apex of shell;
an., anus; ap., aperture of shell; c, columella or axis of shell; e, eyestalk; /, foot;
I, lip of shell; m, edge of mantle, which secretes the shell; r.a., respiratory aperture;
s, suture, between the whorls; t, tentacles. B, the relation of the animal to the shell
when extended.

Questions on the figures What suggestions of bilateral symmetry
are shown by the snail? Where does growth occur in the shell? What
are the functions of the tentacles? What is the function of the edge of
the mantle called the "collar" (m) ?

protrusion from the partly opened shell. This latter power
is much more pronounced in the univalved types (e. g., snail).
The fundamental bilateral symmetry is obscured in the more
sluggish forms, but is very decided in such active animals as
the squid and some of the bivalves.

One of the most interesting points of difference among the
members of the group is the degree of development of the
17

242 ZOOLOGY.

" head." In the bivalves (lamellibranchs) there is a very
slight cephalization, or collection of special organs about the
anterior end. For this reason they are often called Acalephs.
In the gasteropods (snails, etc.) and cephalopods (squid), on
the other hand, the head is well developed both as to special
mouth parts and nervous organs.

The forms with shells are somewhat more limited in size
than the cephalopods, which furnish the largest representatives
of the phylum, measuring in extreme cases 20 to 40 feet in
the reach of the arms.

The calcareous shell insures abundant fossil remains, repre-
sentatives being found in various geologic formations from
the beginning of the Palaeozoic era to the present.

281. Integument (skin). This consists of a layer of epi-
dermal cells, covering a deeper dermal layer derived from the
mesoderm. The former is made up chiefly of the supporting
cells and the simple glandular cells which secrete mucus, or
lime, or pigment. In many forms a large portion of the epi-
thelium in the mantle cavity (as the inner surface of the
mantle and the covering of the gills in Lamellibranchs) is
ciliate. The dermis is a complex of connective tissue, muscle
fibres, pigment cells, etc. The mantle is a fold of the skin
which is very characteristic of Mollusca. It grows out from
the dorsal wall of the body and encloses a space known as the
mantle cavity. It is usually important in respiration, and con-
tains the shell-glands.

282. Shells are formed in all the classes of Mollusca, al-
though naked forms occur in several of them. The shell is
a true secretion or excretion, deposited by the epithelial layer
of the mantle. It consists of three layers : (a) a thin external
layer of organic material known as conchiolin, (fr) the pris-
matic layer, embracing the greater thickness of the shell and
made up of prisms of carbonate of lime cemented by con-
chiolin, and (c) the nacreous or pearly layer over the inner
surface. The edge of the mantle secretes the first and second
layers, and they usually show lines of growth parallel with

MOLLUSCA. 243

the edge of the mantle; the pearly layer is deposited by the
whole surface of the mantle. The point of attachment of the
muscles presents a depression in this layer because the deposit
has been interrupted (see pallial line and muscle scars, Fig.
107; and in shell of clam).

In some Cephalopods there is an internal skeleton in part
secreted by the mantle (cuttle bone), and in part formed of
cartilage (the brain case).

283. The muscular system is made up of unstriped muscle
fibres, which usually occur in more or less prominent bands
or muscles. These may be classified as follows : ( i ) shell or
skeletal muscles, which embrace (a) adductors, those which
draw the valves together (lamellibranchs), (6) rectr actors,
which withdraw the whole or special portions of the animal
into the shell (lamellibranchs and gasteropods), (c) pro-
tractors or extensors, which enable the animal partly to extend
itself; (2) pallial (mantle) muscles, best developed in cephalo-
pods; (3) the foot, which is a mass of muscle and is one of
the most characteristic of the molluscan organs; and (4)
minor muscles controlling the radula or tongue, the other
mouth parts, and the like.

Locomotion in the group is accomplished chiefly by the foot,
in its various modifications, or by rhythmic opening and shut-
ting of the valves. The squid has a fin-like extension of the
integument which is an efficient organ of forward motion.
The siphon of the same animal is regarded as a modification
of a part of the foot. The tentacles about the mouth are also
looked upon as arising from the anterior part of the foot,
hence the name Cephalopod, which means "head-footed."

284. Digestive Organs. Mouth and anus both occur, and
are usually widely separated. In the coiled forms (as the
snail), however, by the looping of the digestive tract they are
brought close together. In all except the group of headless
mollusks (lamellibranchs) the mouth is supplied with a radula,
or tooth-bearing tongue. This lies in the floor of the mouth
and, as it is worn away in front, is renewed from behind in

244 ZOOLOGY.

the radula sac (Fig. 109). It rasps small particles from solids
and conveys them backward into the oesophagus. In the
gasteropods there is a plate in the upper jaw against which
this organ works. In the cephalopods beak-like jaws occur
suited to their carnivorous habit. The mouth is followed by

FIG. 109.

FIG. 109. Diagram of mouth of snail, showing the lingual ribbon (radula). br,
brain; c, buccal cavity; co., coelom; g, gullet; /, jaw, against which the radula works;
m, mouth; r., radula; r.s., radula sac, in which the radula is renewed as it is worn
away in front.

Questions on the figure. What parts go to make up the " odonto-
phore"? How do the parts act in biting?

a gullet, which may communicate at once with the stomach
(lamellibranchs), or may expand into a crop (gasteropods and
cephalopods ) . The stomach is well marked and opens into the
intestine which is usually long enough to make one or more
coils in the body mass. It may open externally (gasteropods)
or in the mantle chamber (cephalopods and lamellibranchs).
Salivary glands pour their secretion into the mouth cavity or
into the gullet, and the so-called liver connects with the
stomach or intestine.

285. Respiration. The oxygen may be derived from the
water (lamellibranchs, cephalopods, and some gasteropods)
or from the air (pulmonate gasteropods). In the latter a
pulmonary chamber is formed by the mantle. Blood is richly
supplied to the walls of this, sac and is there aerated after the
manner of lungs. In the water-breathing forms the gills are
variously constructed. Lamellibranchs possess a pair of " gill-

MOLLUSCA.

plates " hanging in the mantle cavity on either side the body.
These are made up of an immense number of ciliated tubular
filaments which intercommunicate in a complicated lattice-
work. To the naked eye they appear as thin sheets with
striations passing from the dorsal to the ventral margin (see
dissection of clam). The walls of the gills contain blood
vessels, and the water, assisted by the action of the cilia, circu-
lates over and through the gills. In the cephalopods and
aquatic gasteropods the gills occur as tufts of filaments, which
may or may not be covered by the mantle. In addition to
these special organs the mantle and the soft body surface

- m

FIG. 110. Diagram showing the heart and general course of the circulation in the
Lamellibranchs. Only a short section is shown, a, auricle (right), with slit to ven-
tricle; b, the body (region of spaces, lacunae, capillaries); g, the region of the gills, with
capillaries; k, kidneys, with their capillaries; m, the mantle and capillaries; v, the
ventricle from which arteries pass forward and backward; v.c., "vena cava," in which
the blood collects on returning from the tissues of the body.

Questions on the figure. Follow by the arrows and letters the general
course of the blood flow. How many sets of capillaries are passed by the
blood which goes to the mantle? By that which goes to the system, be-
fore returning to the heart? What changes take place in the blood in the
capillaries of the various regions ?

246

ZOOLOGY.

assist in respiration. (For figures of the gill structure in the
clam see Parker and Haswell's Text-book of Zoology, Vol. I,
Fig. 5 2 9-)

286. Circulation. There is usually a well-developed circu-
lation of the blood, but a portion of it occurs through irregular
spaces devoid of proper walls. The organs consist of a con-
tractile heart . usually with undivided ventricle and a single
auricle (gasteropods), or one pair of auricles (lamellibranchs,

tissues

tissues

FIG. in. Diagram showing the general course of the circulation in mollusks. Com-
pare with Fig. no, which shows the organs more nearly in their relative position.

Questions on the figure. Why does the blood which passes to the
mantle not need to pass to the gills before returning to the heart ? What
happens to the blood in each of the regions named in the diagram?

squid), or two pairs (Nautilus}. Definite arteries pass both
forward and backward from the ventricle. The blood passes
from the ventricle to the tissues of the body, whence it gathers
into venous spaces and passes into the kidneys and the gills.
From the gills it finds its way to the auricles. In lamelli-
branchs the blood x \vhich goes from the ventricle to the mantle
returns directly to the auricle. In some Cephalopods there
are branchial hearts near the gills to assist the return of- the
blood to the heart. The accompanying diagrams (Figs, no,
in) will help the student follow the main facts of the circula-
tion. In lamellibranchs the ventricle often surrounds the in-

MOLLUSCA. 247

testine. The corpuscles are colorless and amoeboid. The
plasma, however, quite commonly contains a bluish pigment
(haemocyanin) which assists respiration in somewhat the same
way as the haemoglobin of the vertebrates.

287. Excretory Organs. In mollusks the excretory
organs consist, when reduced to the simplest terms, of one or
more nephridia which communicate interiorly with the peri-
cardium or principal ccelomic space, and with the exterior by
way of a tubular ureter. The kidney portion of the tube is
much modified, has glandular walls and is well supplied with
blood vessels. It lies in the immediate region of the peri-
cardial chamber in most cases.

288. Nervous System. The nervous system of mollusks
is usually made up of at least three pairs of ganglia: (a) the
" brain " or cerebral ganglia dorsal to the mouth and varying
in size according to the degree of development of the head ;
(6) connected with the brain by a pair of connectives are the
pedal ganglia lying ventral to the mouth and innervating the
foot; (c) the pleuro-visceral ganglia variously situated in the
different groups and connected with the brain or both with
the brain and the pedal ganglia. From it nerves pass to the
mantle, and to the posterior organs. In gasteropods and
cephalopods these ganglia are much closer together and are
collected about the mouth. Still other ganglia are often asso-
ciated with them. The student should notice how this collec-
tion of nervous matter accompanies the development of
" head " organs in the better developed members of the
phylum.

289. The Organs of Special Sense. As usual, scattered
sensory cells are situated in the exposed epithelial surfaces.
These give rise to a diffuse sensitiveness to tactile and chemi-
cal stimuli. The edges of the mantle and the tentacles are
especially sensitive. Patches of sensory cells osphradia
are often found near the bases of the gills, which probably
have a value in testing the character of the water flowing over

2 4 8

ZOOLOGY.

them. Still other patches occur about the lips. Otocysts (see
1 08) occur in all the groups. Eyes are usually found and are
of various degrees of complexity. They are simplest in the
lamellibranchs (Fig. 41), and when found at all in this group
may occur in great numbers along the mantle edge. In the
gasteropods the eyes are borne on the ends of tentacles and
are frequently destroyed by accidents. The animals have the
power of regenerating the tentacle, eye and all. This mani-
festly is a very useful adaptation. The eyes of cephalopods
are the most perfect single eyes found among the invertebrates.

FIG. 112.

FIG. 112. Diagram of a dissection of the reproductive organs of a snail, a.g., albu-
men gland; c.d., common or hermaphrodite duct; e.g., hermaphrodite gland; d.s., dart
sac; f, flagellum; g, genital aperture; m.g., mucous glands; o, oviduct; p, penis; r.s.,
receptaculum seminis; v.d., vas deferens. The slit from the genital aperture into the
oviduct and penis shows the openings of the dart sac, mucous glands, and the recep-
taculum seminis. (After Pelseneer.)

Questions on the figure. By a careful study of the figure and the
text, determine the functions of the various parts of the system. Does self-
fertilization occur in a form like this? Evidences.

MOLLUSCA. 249

Though originating in a different way, it is strikingly like
the vertebrate eye.

290. Library Reference. Make a report on the position and general
structure of the eyes in gasteropods, cephalopods and lamellibranchs.

291. Reproduction and the Genital Organs. Reproduc-
tion is always sexual. In some of the lamellibranchs (e. g.,
oyster) and many of the simpler gasteropods, including the
land snails, the individuals are hermaphrodite. The sexes are
separate in the cephalopods and in most of the lamellibranchs
and gasteropods. The organs are more complicated among
the hermaphrodite gasteropods than elsewhere in the group
(see diagram reproductive organs of snail, Fig. 112). The
sexual glands are usually situated in the visceral mass among
the coils of the intestine. The ducts ordinarily open into the
mantle cavity where fertilization may occur. The eggs after
fertilization are often, either singly or in masses, surrounded
by a gelatinous secretion (as in the snail) which serves as a
protection from drouth and as a means of attachment. In
lamellibranchs the young are not infrequently retained in the
mantle or respiratory chamber until partly developed.

292. Development. Segmentation is total (lamellibranchs
and gasteropods) or partial and discoidal (dibranch cephalo-
pods). It is usually unequal in the lamellibranchs and gastero-
pods, but in some of the latter it is equal during the first two
divisions, producing four equal blastomeres. Each of these
divides into a large and a small cell macromere and micro-
mere. Still other micromeres are formed at the expense of
the macromeres, and these by continued division form a cap
of ectodermal cells (Fig. 113). From the macromeres arise
ultimately the entoderm and mesoderm. The gastrula may
be formed either by invagination of the large cells or by the
overgrowth of the micromeres, depending on the size of the
segmentation cavity and of the entodermal cells. In the
cephalopods, owing to the large supply of food substance in
the ovum, cleavage is confined to a small disc at the active pole.

250

ZOOLOGY.

From this point where the embryo is destined to be developed,
a sheet of cells gradually extends itself by growth around the
yolk. Thus a yolk-sac is formed by means of which the food
is used in the further development of the embryo. By the time
the embryo is hatched the yolk is exhausted. Although the
yolk does not segment we see that it serves its purpose in the

FIG. 113.

mi.

mes.- rtr^

ma.

FIG. 113. Diagram of early segmentation stages in a Gasteropod. A, 2-celled stage;
B, 4-celled; C, 8-celled; D, later stage, in section, ect., ectoderm cells (micromeres) ;
ent., entoderm cells, macromeres; tncs., mesoblasts, early put aside, before gastrula-
tion to form the mesoderm; mi., micromeres; ma., macromeres.

Questions on the figures. What causes are assigned for the differ-
ence in the size of the cells in the 8-celled stage ? In what other ways is
mesoderm formed in the metazoa ? Which cells seem to divide more
rapidly, the micromeres or the macromeres? Compare with Annelid,
Fig. 103.

development of the embryo. The later development is typi-
cally indirect, i. e., with a metamorphosis, though many (as
the cephalopods) develop directly into the adult form. A
larval stage (trochosphere} occurs, suggesting the larvae of
the Polychseta. This is followed by another stage (veliger}
which is more characteristic of the Mollusks.

293. Library Exercises. Appoint students to supplement the text by
making short reports on the following topics : the early segmentation of
lamellibranchs and gasteropods ; of the cephalopods ; the veliger of
mollusks ; the formation of the organs in cephalopods ; development in the

MOLLUSCA. 251

clam or mussel. Illustrations should be found in the advanced text-books
and presented to the class.

294. Ecology. The bivalves are sedentary or sluggish in
their manner of life; the motion of most of the gasteropods
is slow and difficult. In conformity with their limited powers
of locomotion, they are scavengers, feeding on the debris and
the small animals and plants brought to them by the water
currents (oysters, mussels, etc.), or are largely herbivorous
(many snails). A very few are parasitic. The cephalopods
are much more active and are carnivorous. For the most part
the sluggish forms are well protected by the shells, neverthe-
less they furnish food for many diverse sorts of animals.
Some of their enemies are internal parasites, others bore
through the shells and thus gain access to vital parts.

The animal within may thwart this attack of its enemies by
the continued secretion of mother-of-pearl on the inner sur-
face at the threatened point. Some animals crush the shells,
or swallow the mollusk, shell and all. Star-fishes, as we have
seen, are especially troublesome to the oyster beds.

Many of the bivalves are capable of still further protection
because of their elongated siphons which enable them to
burrow deeply in the mud or sand, the food being carried in
through the siphons by the water currents (Fig. 1 14). Several
species of marine bivalves have the power of boring into wood
or even stone. This serves as a protection to them, but often
results in the complete destruction of piles and other structures
placed in the ocean by man.

Many of the mollusks seem more or less gregarious, as is
illustrated by beds of clams and oysters, the schools of squid,
etc.

Notwithstanding the low organization and sluggishness of
a large portion of the branch Mollusca, we are compelled to
consider that it has been a very successful group in that it
has held its place with practically equal numbers through the
geological ages, and has succeeded in adapting itself to the
changes of those ages. Of no less interest is the additional

252

ZOOLOGY.

fact that there is scarcely a nook into which they have not
penetrated, except where continuous drouth prevails. On the
other hand, it is among the more active types the cephal-
opods that the ancient geological forms have least success-
fully adapted themselves to modern conditions. The cephal-
opods appear much less numerous and varied now than in
earlier geological time.

295. Classification. The following are the principal classes :
Class I. Pelecypoda or Lamellibranchiata (Mussels, Oysters, etc.).
Lamellibranchs are mollusks in which the fundamental bilateral sym-

FIG. 114.

FIG. 114. Mya arenaria, a burrowing clam. The siphon is represented as fully ex-
tended. This is quickly retracted when the animal is disturbed. (After Kingsley.)

Questions on the figure. What is the function of the much elongated
siphons? Which is the anterior end of the animal? Which the dorsal
side? What would seem to be the chief function of the foot in this case?

MOLLUSCA. 253

metry is shown in the right and left valves of the shell secreted by a
bilobed mantle, and in some of the internal organs. There may be one
or two adductor muscles. The head is undeveloped. The ventral body
region is differentiated into a muscular foot, shaped like a plow-share.
The gills are in sheets (see 285) usually two on either side, and are sus-
pended in the mantle cavity. Paired labial palps occur about the otherwise
unspecialized mouth. The three pairs of ganglia, the cerebro-pleural,
the pedal, and the visceral, are usually well separated. The heart con-
sists of two auricles and one ventricle surrounded by a pericardial space,
which is a portion of the body cavity and communicates with the exterior
by a pair of nephridial tubes. The reproductive organs are simple; the
sexes are ordinarily separate. Development by a metamorphosis.

[The primary subdivisions of the group may be made on the basis
either of the gill structure, the adductor muscles, or the presence or
absence of the siphon.]

Order I. Isomya : Two adductor muscles which are essentially equal.

(o) Siphon well developed, retractile; pallial line (Fig. 107) with a
sinus. Here occurs Mya arenaria, the common clam of the Atlantic coast.
Great heaps of shells of this clam show that it was much used by the

FIG. 115.

FIG. 115. Ensis americanus, the razor clam. From Verrill, after Gould.

Questions on the figure. Where is the hinge, the umbo, etc. ? Trace
the lines of growth and compare with other figures of bivalves.

Indian tribes as food. In New England the clam fisheries are of very
considerable importance. Mya burrows in the mud, using its long siphon
to keep it in connection with the water from which it gets its food. Of
somewhat similar habits is the razor-shell clam (Fig. 115). The "borer"
(Pholas} and the "ship-worm" (Teredo} belong to this group and
possess the power of boring into wood or stone and are thus of much
damage to submerged structures in waters where they abound.

(6) Siphon usually present but not highly developed; no pallial sinus.
In this group are embraced the more abundant fresh water mussels (Unio,
Anodonta, Cyclas), and the cockles (Cardium} of the ocean. The
Unionidse are very widely distributed and very common in our own fresh
waters. They are not much used for food at present, though the Indians
used them, probably in times of scarcity of other food. Their shells are
widely employed in the making of buttons, knife handles and the like,

254

ZOOLOGY"

and pearls of value are not of infrequent occurrence. These are merely
the mother-of-pearl, which ordinarily lines the shell, secreted about a grain
of sand or other irritating object which finds its way between the mantle
and the shell. Great quantities of these pearls are sometimes found in
the graves of the mound builders.

FIG. 116.

FIG. 116. Mytiius edulis, a Mussel. From Binney's Gould.

Questions on the figure. Identify the umbo. What are your evi-
dences that it is the umbo? Compare the lines on the shell with those in
figure 117. What is the significance of the specific name (edulis) ? What
are the habits of the species?

Order 2. Heteromya : Two adductor muscles, the anterior much re-
duced ; siphon usually wanting. Here are included the horse-mussel
(Modiola) and Mytiius, edible mussels which occur in clusters just below
low tide mark ; also the pearl-oyster, from which the best pearls are taken.
The last mentioned form is not found on our own coasts.

Order 3. Monomya : One adductor muscle (posterior) ; no siphon.
The genus Ostrea (oyster) and the genus Pecten (scallop) are the most
interesting and important representatives of this order. The species of
Ostrea differ much in size in different regions. The largest living species
is a Japanese form which is known to reach a length of two to three feet.
The oyster is hermaphrodite. The young, after a short free life, become
attached by one of the valves. The oyster constitutes a larger element in
the food supply of man than any oiher invertebrate. The scallops are
not attached, and swim by a rapid opening and closing of their valves.

Class II. Gasteropoda (Snails, Slugs, Whelks, and Periwinkles').
Gasteropods are mollusks with unsymmetrical, univalved, usually spiral
shells (occasionally lacking the shell altogether). The head and foot

MOLLUSCA.

255

ordinarily preserve the bilateral symmetry, but the other organs lose their
symmetry both from the spiral form of the shell and from a twisting
which many of the forms undergo by which the nervous system and
certain other visceral organs lose their original right and left relations.
The head region is well developed, having tentacles, eyes, and a mouth
with a tooth-bearing radula. Gills in the mantle cavity two, one, or none ;
in the air-breathing forms there may be merely a pulmonary sac. The
sexes are separate (Streptoneura) or united in one individual (land snails).
Development is mostly indirect.

FIG. 117.

FIG. 117. Pecten irradians, a Scallop. From Binney's Gould.

Questions on the figure. Is this an external or internal view of the
shell? Where is the umbo? What is peculiar about the hinge in this case?
What is the significance of the lines nearly concentric with the margin?
Of the radial lines?

Subclass I. Strcptoncura. Gasteropods in which the nerve loop made
by the visceral commissures, is twisted in development into the form of
the figure 8; the other visceral organs are twisted so that right and left
are interchanged. Only one pair of tentacles on the head. Sexes separate.
Gills usually in front of the heart.

One of the common representatives of this group is Llttorina, the
common periwinkle of the seashore. Many other types of almost infinite
variety of form, size, and color inhabit the ocean, their shells often being
washed ashore by the waves ; such are the cowries, the whelks, the cone-
shells, etc. Here belong the uncoiled Limpet and the slightly coiled
Crcpidula or boat-shell.

256

ZOOLOGY.

Subclass II. Euthyneura (Land Snails and many naked Mollusks).
Gasteropods in which the nerve loop is not twisted. The head usually
bears two pairs of tentacles. The sexes are united in the same individual.
The most important of these are the Pulmonata or air breathing Gastero-
pods, some of which are terrestrial and others aquatic. Of the terrestrial
snails the genus Helix (Fig. 119) is the most widely distributed and inter-
esting. Its variability is such that between three and four thousand

FIG. 118.

FIG. 119.

FIG. 118. Acma-a testudinalis (Limpet), from Binney's Gould. Upper figure lateral
view; lower figure, dorsal view.

Questions on the figure. How do the Limpets differ from the ma-
jority of the snails? What is the appropriateness of the specific name
(testudinalis') ?

FIG. 119. Helix albolabris, a pulmonate Gasteropod. From Binney's Gould.

Questions on the figure. What is the significance of Helix? Of
albolabris ? Identify the parts of the shell. Is it a right or left spiral?
What do you mean by your answer?

FIG. 1 20.

FIG. 120. Limax flai'us, a Slug. From Binney's Gould.

Questions on the figure. How do the slugs differ from the other
Gasteropods? In what external respects do they appear similar to them?
Compare all the figures of slugs you may be able to find.

MOLLUSCA. 257

species have been described. Limax (Fig. 120) is a pulmonate form in
which the shell is practically wanting. It is especially destructive to cer-
tain types of plants as it is a voracious vegetable feeder. The aquatic
pulmonates are represented by the "pond-snail" (Limnaa), and by
Planorbis, a snail whose coils are in one plane, presenting a helix rather
than a spiral.

Class III. Cephalopoda (Squid, Devil-fish). The cephalopods are
bilaterally symmetrical mollusks with a well-developed head in which

FIG. 121.

e'

FIG. 121. Pearly Nautilus. From Nicholson, e, eye; h, hood, a muscular portion
of the foot which protects the softer parts; s, siphon; se, septa, separating the succes-
sive chambers of the shell; sp, siphuncle; t, tentacles.

Questions on the figure. How does this shell compare with those of
the Gasteropods? What is considered to be the homology of the tentacles
or arms in Cephalopods? What is the siphuncle? What is the character
of the eye in Nautilus?

the front part of the foot surrounds the well-armed mouth as a series
of lobes or tentacles. The head protrudes permanently from the mantle
cavity, leaving the mantle surrounding the posterior part of the body. The
posterior lobe of the foot forms a siphon, communicating with the
mantle cavity. Into this cavity the nephridia, the anus, and the reproduc-
tive glands open, and in it the gills lie. The shell may be present and ex-
ternal (Nautilus}, internal and slightly developed (Squid), or wanting
(Octopus). An internal cartilaginous skeleton protects the brain. The
coelom is well developed. The ganglia of the nervous system are massed
in the head region. The sexes are separate and the development direct.
The Cephalopoda are to be looked upon as the most highly developed of
the Mollusca. They are little in evidence now, however, as compared with
earlier times.

Subclass I. Tetrabranchiata. Cephalopoda in which the front segment
of the foot is divided into lobes bearing numerous tentacles, without
18

2 5 8

FIG. 122. The Devil-fish (Octopus). From Cooke, after Merculiano. A, at rest;
B, swimming, a, arms, with suckers on the inner aspect; e, eye; s, siphon or
funnel.

Questions on the figure. Which is the anterior end of the animal?
What is the position of the mouth? What is the function of the siphon?
Of what structure is it a part?

FIG. 123.

Fie. 123. The Paper Nautilus (Argonauta argo~). From Cooke, after Lacaze-Duthiers.
e, eye; m, mouth; /, siphon; sh, shell; t, tentacles.

Questions on the figure. In what way does the siphon serve in loco-
motion? In which direction will the animal move by means of the
siphon? How does the shell of Argonauta differ from that of Nautilus?

MOLLUSCA. 259

suckers. Shells external and chambered (and in Nautilus, the only living
genus, coiled). Two pairs of auricles; two pairs of gills; two pairs of
nephridia.

This group is important' for its extinct rather than for its living repre-
sentatives. The pearly or chambered nautilus (Fig. 121) found in the
Pacific and Indian Oceans, is the only important living species. The
Nautilus appears to be the only remaining descendant of the once numer-
ous family of Ammonites and more remotely still of the Orthoceratites, the
rulers of the Palaeozoic seas (see Geology).

Subclass II. Dibranchiata. Cephalopods in which a circlet of 8 to 10
arms surround the mouth. These bear sucking discs. Shell internal and
rudimentary^or absent. One pair of gills, one pair of nephridia, and one
pair of auricles. An ink gland is present.

Order I, Decapoda, embraces the cuttle-fish and squid.

Order 2, Octopoda, embraces the devil-fishes (Fig. 122) and the paper
nautilus (Fig. 123).

296. Supplementary Studies for Library, Laboratory,
and Field.

1. Compare the clam, snail, and squid with regard to the
following particulars, putting the results in a tabular form :

(a) Degree of development of the head.

(&) Shell, development and method of using, in each.

(c) Mantle; extent, form and modifications: mantle cavity.

(d) Foot; parts, differentiation, and uses.

(e) Respiration; how accomplished?

(/) Sense organs; position, character, and degree of de-
velopment.

(g) Locomotion ; N how effected?
(h) Protection; special devices.

2. Can you find any indication among the mollusks of a
relation between the degree of development of the sense
organs and the activity shown by the animals? Between the
external protective structures and activity?

3. When did the various classes of mollusks make their
appearance in the history of the earth? (See geology.) What
can you say of their importance in the formation of the sedi-
mentary rock?

4. In what ways may the fresh-water forms have arisen
from the original salt-water mollusks ?

26O ZOOLOGY.

5. What members of the group of mollusks are economically
important ? Indicate in what way and to what extent ?

6. A report on all the mollusks to be found in your com-
munity; their distribution, habits, etc.

7. Formation of pearls, and pearl fisheries.

8. The industries connected with the use of the shells of
the clam.

9. The life history of the fresh-water clam.

10. The life history of the oyster.

CHAPTER XVII.

PHYLUM VII. ARTHROPODA.

297. This group is one especially favorable for the pupils
to study in the field, in the haunts of the animals themselves.
For this reason, wherever it is at all possible, the members of
the class should be required to collect a portion of the material
needed in the laboratory and to submit a report on such items
of physiology and ecology as may be expedient in each case.
The teacher will find suggestions in the supplementary
exercises.

298. The Fresh-water Cray-fish (Cambarus). This form
should be studied when living specimens may be had. They
may be kept for considerable time in a tub containing an inch
of water. This should be changed every day or two. Feed
on small pieces of meat or earthworms.

I. Physiology.

1. Locomotion: walking; how effected? Swimming; how
'effected? Under what circumstances does the animal swim?
Do all the walking legs act together in walking? How many
are at rest at once? In what order do they act?

2. Movements of the parts of the body: segments, and
appendages. Describe the manner and purpose of these mo-
tions as far as you can determine. In what different ways do
the various groups of appendages seem to act? Watch them,
one pair at a time.

3. Feeding: kind of food used and manner of securing it.

4. Respiration: by means of air or water? How can you
be sure? Does the animal do anything to renew the water,
by producing currents? Place a minute amount of carmine
or indigo solution at the side of the animal at the union of
the abdomen and thorax; at the front of the thorax. What is
the difference? What does it signify?

261

262 ZOOLOGY.

5. Evidences of sensitiveness : Devise experiments of your
own to prove whether the cray-fish is stimulated by light;
contacts; the presence of food in any other way than by sight;
sound. Are all parts of the body equally sensitive to touch?
To chemical stimuli? Make use of a 5 per cent, solution of
acetic acid; strong salt solution; strong beef extract. What
inferences may be drawn from your experiments?

II. Symmetry. (This group is especially favorable for
this study.)

Notice what is implied in bilateral or tri-axial symmetry.

Antero-posterior axis: are the poles alike or different?

Make a memorandum of all the chief differences.

Dorso- ventral axis (as above).

Right-left axis. Record the points of agreement.
Contrast the axes in length. Can you think of any causes

for the differences and likenesses discovered above? Any

advantages arising therefrom?

III. General Form. Distinguish two regions; Cephalo-
thorax and abdomen.

Cephalo-thorax ; carapace.
Head ; rostrum, eyes, mouth.
Cervical groove.
Thorax.

Abdomen; how many segments do you find? What seems
to determine a segment?

Applying these criteria can you find any indications of seg-
mentation in the cephalo-thorax ? (Make a temporary
estimate of the number of segments in the animal.)

Make two sketches showing a dorsal and a ventral view of
the cray-fish, preserving proportions.

Examine one of the abdominal segments (the third or
fourth from the front). How is it joined to those next
it? Follow the line of union. Note, tergum, or dorsal
piece; sternum, or ventral piece; pleura, the lateral pro-
jections from the tergum.

ARTHROPODA.

263

Make a sketch of an imaginary cross-section showing the
relation of these parts to each other, together with the
attachment of the appendages.

IV. Appendages. Group them into regions and notice the
general differences and the differences in the uses to
which they are put. If time will allow, study the ap-
pendages in detail as follows :

1. Begin with the third or fourth abdominal appendage (swimmerets)
making the drawings necessary to show the parts :

Protopodite, or basal portion.

Exopodite, or external branch.

Endopodite or internal (median) branch.

Compare all the abdominal segments with that studied. Do different
individuals agree in the appearance of the first and- second abdominal
segments? Compare the last segment (telson) with those studied. How
many segments in the abdomen? Of what parts is the tail fin made up?

2. Cephalo-thoracic Appendages. Remove with scissors the over-
arching portion of the carapace and expose the base of the appendages.
Find the third maxilliped (the first appendage in front of the large claw).
Remove by inserting a scalpel and bringing away all that belongs to it.

Identify :

Protopodite, of two segments (coxopodite, next the body, and

basipodite).

Endopodite and exopodite. How many pieces in each?
Epipodite, lying in the gill-chamber. Are there any special out-
growths on it?

Study and compare with this the large claw, and the other walking
appendages. Which part is wanting in these, exopodite or endo-
podite? Reasons for your view? How do these five appendages
differ from each other.

Examine and compare the appendages in front of the third maxilliped
in order :

Second maxilliped.

First maxilliped.

Second maxilla.

First maxilla.

Mandible. f Head P arts -

Antenna.

Antennule.

What are the evidences that the antennae and antennules are homolo-
gous with those already described?
Revise your estimate of the number of segments.

264 ZOOLOGY.

Compare the appendages again by groups, and notice the chief points
of difference, and the ends served by these differences. Make a
careful sketch of each type of appendage, labeling all parts. (The
names of the segments of the larger appendages may be found in
fuller texts, if desired.)

By studying the living specimen, determine just the work done by
each of the types of appendages.

Note the position of the eyes. Examine with a low power.

In the basal joint of each antenna is the opening of the " green gland."

In the basal joint of the antennules are the otocysts.

V. Gills. Examine the gill-chamber, and the position of
the gills therein. Which appendages bear gills? How many
tufts to each appendage? How do they differ as to the place
of their attachment? How many in all? Make a table showing
these facts.

VI. Internal Organs. Remove with much care the carapace from the
thorax and the terga from the abdominal segments, by the use of scissors
and forceps. Sketch the organs in their natural position. What organs are
visible?

Examine in some detail the following sets of organs,
(a) The circulatory organs.
Heart : just beneath the carapace, in a membranous chamber (peri-

cardial sinus).
Apertures, by which the blood enters the heart from the sinus;

dorsal, ventral, lateral. How many do you find?
Arteries ; anterior, posterior.
(The teacher should have, if possible, a permanent preparation of the

lobster in which the arterial circulation has been injected with a

colored mass.)

VII. Reproductive Organs. These will be found immediately beneath
the pericardial sac as whitish (male), or yellowish to brown (female)
lobed structures. Depending on the sex there will be found

Ovaries or testes. Form, position, and number of lobes?
Oviducts or vasa deferentia. Course, length and outlets?
Can you determine the sex of your specimen? Note especially the ex-
ternal differences between males and females.

VIII. Digestive Organs.

Liver, a pair of yellow, brown or reddish masses anterior to the re-
productive organs.

Stomach ; sketch in position. Dissect later, if time allows, and note
the anterior and posterior chambers, and the grinding apparatus.

How is the mouth situated relatively to the stomach?

Follow the intestine backward from the stomach to the
Anus: position of?

ARTHROPODA. 265

Make a sketch of the entire tract from a side view, showing in what
part of the carapace each portion is.

IX. Muscular System. How is the abdomen flexed and how extended?
How do the muscle fibres run? To what attached? Are they plain or
striate? How are the appendages worked? Split open the segments of the
chela.

X. Nervous System. (If the time is short a demonstration may be
made by the teacher, preferably with a lobster.)

Remove the intestine, and cut carefully through the muscles in the
median line until the white ventral nerve-chain is uncovered. Follow it
forward to the head, cutting away the covering plates in the thorax.

How many swellings (ganglia) in the abdominal region? Relation to
the segments ? Where do nerves arise ?

Thoracic ganglia : number and relation to appendages ?

Subcesophageal ganglion; circumcesophageal connective.

Supraoesophageal ganglion (brain).

Do any nerves arise from the brain? Where distributed? Draw from
above.

Make a diagram of the relation of the digestive tract and nervous sys-
tem from the side view.

XI. Excretory Organs. The green glands occur at the base of the head,
in front of the mouth. The outlets are at the base of the antennae.

Make a diagrammatic view of an imaginary cross-section of the thorax
in the region of the heart, and one of the abdomen, showing the position
of the internal organs. Also a diagram of a sagittal section showing rela-
tions of all the parts discovered.

. 299. Sowbug (Oniscus, a terrestrial form; or Asellus, a fresh water
Isopod).

General Form. Use hand lens and identify :
Head : size, form, number of segments.

Eyes : number and position.

Antennules and antennae.

Mouth-parts : number and structure.

Thorax : number of segments. What variation therein ? '
Abdomen. How many segments? Proofs?
Appendages.

Remove carefully, mount in water on a slide, and examine with low

power, a thoracic appendage. Sketch.
Examine similarly the other thoracic legs and the mouth parts, and

make drawings of them arranged in the order of their occurrence.
Examine similarly the abdominal appendages. What is their number?

Sketch.
Compare the appendages from the different regions, as to structure,

form and probable function. Are there any gills? Where situated?
What is the number of segments in the body, if there is a pair of

appendages to each segment?

266 ZOOLOGY.

Comparisons. 'Compare the sow-bug with the cray-fish as to the
degree of union of head and thorax ; the number of segments represented
in each of the three regions ; the degree of differentiation among the
appendages; the mode of respiration; the presence of both exopodite and
endopodite ; as to food, and habits.

Physiology and Ecology. A study and report of the animal's habitat,
food habits, methods of motion, sensitiveness to light and to other classes
of stimuli, should be made. How does Oniscus behave when touched?
Do you find any trace of eggs or young? What facts are to be noted con-
cerning them?

300. Cyclops. -These minute freshwater Crustacea may be found in
almost any pool where aquatic plants are found. They flourish well in
aquaria. Select several of the larger specimens with egg masses one on
each side the abdomen. Examine in a watch glass with a little water to
which a drop of chloroform has been added. Use low power of microscope.

General Form. (Study both dorsal and ventral surface.)
Cephalo-thorax :

Anterior portion covered with the carapace. How many segments
represented ? How can we know that this is not merely the head,
or the whole cephalo-thorax ?
Posterior portion (four free thoracic segments). How is it known

that these are not abdominal segments?
Abdomen: form; number and character of the segments.
Appendages. Antennae, oral, thoracic, abdominal. Number and general
character of each. Where and how are the egg-cases attached?
Sense Organs.

Eye spot (appearing as one, from which the name Cyclops).
Do you find any organs which suggest a tactile function?
Report on all available points of physiology: as food habits; methods
of locomotion ; reaction to light and other stimuli.

301. Comparisons. Collect all the minute fresh-water Crustacea pos-
sible and compare them with Cyclops. Learn to identify them by their
manner of moving in the vessels of water. Daphnia is especially favorable
for microscopic study on account of its semi-transparency.

302. Spider (any common species large enough for study).
General Form. Study the relations of head, thorax, and

abdomen. Are there any antennae? Oral appendages? Num-
ber and character of the thoracic appendages? Does the ab-
domen show any signs of segmentation? Has it any append-
ages? Make sketches showing a ventral and a lateral view.
Special Organs.

Examine the head with a hand lens and locate the eyes.

Note more particularly the types of appendage found,

ARTHROPODA. 267

and the degree of differentiation. Find the openings
to the air-sacs on the ventral surface of the abdomen.
Locate the spinning glands. Number?
Activities and Habits. How do the legs act in walking?
At what joints are they flexed at various parts of the step?
Do all the legs on one side act in unison? Observe the spin-
ning action. Does the spider ever produce the threads except
when weaving a web? Describe. Determine if possible the
kind of web formed by the species studied. Or find as many
types of nest or web as possible and learn to recognize the
spiders producing them. How does the spider travel on its
web ? Where do spiders place their webs ? Place a fly or other
insect in a newly constructed web and record the actions of
the spider. Can you devise means to prove whether the spider
possesses the sense of smell?

303. The Grasshopper. Several species of the locusts
may be found in almost every locality. They are especially
abundant in the early autumn. For laboratory study select
the largest species found in sufficient abundance. In connec-
tion with the securing of material the students should make
observations on the following points:

1. Habits. Where and under what circumstances found?
At what time of the year does this species occur in greatest
abundance ? Under what circumstances are they most active ?

2. Methods of Locomotion. How many methods seem
available? Degree of efficiency of each ? Under what circum-
stances is each used? What distance can be attained at one
effort? Continue the study later in more limited quarters, as
in the room and under a bell-glass. Compare the work of
the various legs. Are the wings used at all in jumping?

3. Protective Features. Coloring; to what extent do you
find this of protective value ? Reasons. Does the animal show
a distinct instinct for hiding? Compare all available species
in these regards.

4. Do they produce definite sounds? Under what circum-

268 ZOOLOGY.

stances? Do you find any hint as to the method of their pro-
duction ?

5. Do you detect any movements which suggest respiration?
Rate? (Find spiracles in the thorax and abdomen.)

6. Supply hungry animals with fresh leaves ^nd study the
feeding process. Dip the leaves in various solutions and
notice whether it makes any difference to the grasshopper.

If alcoholic material is used for the following morphological
studies it should not be allowed to become dry. If dipped in
a mixture of glycerine and 50 per cent, alcohol, specimens
will not dry so rapidly.

The sexes differ, particularly in the abdominal region.
Procure specimens thus differing by examining a number of
individuals, and keep both kinds for comparison. Sketch
dorsal, lateral and ventral views of each (especially in the
regions of difference).

External Features. Study the following points :

1. The regions of the body.
Head; thorax; abdomen.

What are the signs of segmentation in these three
regions? Where is it most clearly indicated? Where
are the segments most similar?

2. Abdomen.

Number of segments (differs in male and female).
Dorsal and ventral plates. Are they equally developed in

all segments.
Appendages : which segments possess them ?

Ovipositors (paired outgrowths found only in the

female).

Anal cerci (examine the male). Are they found in
the female? To what segments do these appendages
belong?
Spiracles (small openings at the side of the segments) ;

number and distribution?

Tympanic membrane, at the sides of the first abdominal
segment.

ARTHROPODA. 269

3. Thorax ; studying from the front, backward, find :
Prothorax; mesothorax; metathorax. Note the form,

size, and structure of each part.
Appendages of each segment.

Legs: number; relative size; parts (beginning at the
body), coxa, trochanter, femur, tibia, tarsus. Com-
pare the legs.

Wings (can these be regarded as homologous with
the jointed appendages?) : number; position, at rest
and in motion ; characteristics ; position of veins.
Compare the two pairs in all essential particulars.
Are there any spiracles in the thorax? Position?

4. Head (is there any "neck"?). The head is covered
with chitinous plates; identify:

Epicranium, the dorsal plate.

Clypeus, the anterior plate.

Gense, the lateral plates.

Labrum or upper lip, anterior to the clypeus.
Examine the compound eyes, their form and relation to the

plates. Slice off a portion of the surface and study the

surface with a low-power objective.
Ocelli or simple eyes. How many and in what position?
Mouth aperture; position.
Appendages of the head :

Antennae, near the eyes; number.

Mouth-parts. These are complicated and demand careful study, if
satisfactorily made out. Remove the labrum and proceed from
before, backward.

Mandibles; a pair of horny tooth-bearing jaws. Draw in position.

Maxilla; a pair of compound jointed organs made up of three

portions, the lacinia (nearest the median line), the galea, and

the maxillary palpus (external).

Labium or lower lip ; this also bears a palpus. The labium may

be studied and removed before the study of the maxillae.
Tongue.

How many segments seem to be represented in the head?
Internal structure.

Select large female specimens preferably. Clip the wings close to
the body, and pin the specimen to a board, dorsal surface upward.

270 ZOOLOGY.

With a pair of fine, sharp pointed scissors make a longitudinal
incision into the integument of the abdomen near each side.
Gradually and carefully remove the skin between the cuts from
behind forward. Look for the heart, a long, thin-walled, mid-
dorsal vessel, which if not removed with the skin may be seen
just beneath it. Unroof both the abdomen and thorax. Note the
exposed muscles of the thorax, also the whitish fat bodies next
the body wall.

1. Trachea. If the specimen is freshly killed the tracheae will be
filled with air and will show as white, glistening tubes. Seek their
connection with the spiracles, and note their ramification and
unions in the body. Isolate some of the smaller branches and study
under the microscope. Prove that they are tubes. How kept open?

2. Reproductive Organs. (These are much more difficult in the

male.)

Ovaries : In how many masses ? Notice the subdivisions of the
ovaries. These contain the eggs and communicate by means of
an oviduct with the outside. In what segment? Examine an
ovum with the microscope. Mash, and notice the yolk.

3. How do the muscles of the thoracic region differ from those in
the abdominal? Are the fibres plain or striate?

4. Digestive Tube.

Dissect forward into the head, and press the other organs aside so
that the course of the tract may be revealed. It consists of
the following parts, which should be identified.

Mouth.

(Esophagus ; size and course.

Crop (an enlargement of the oesophagus) ; shape, position.

Stomach: character and extent. (At the anterior end is a ring
of tubular appendages which are glandular in function, the
gastric caeca; at the posterior end it is limited by a circle of
fine tubes Malpighian tubules which are excretory.)

Intestine; length, course and size.
Anal opening; position.
Make drawing of digestive tract from side view, showing in outline

the body regions and the relation of the portions of the tract to

these.

5. Nervous System. (Remove the alimentary tube and examine the

floor of the abdominal cavity.)
Ventral nerve cord. Is it single or double?

Ganglia; number, and relation to the segments.

Nerves ; origin and distribution.
Trace forward into the thorax and head.

Ganglia; number and position. How connected? Is there any
portion dorsal to the digestive tract (brain) ?

Nerves.

ARTHROPODA. 271

Compare the nervous system of the grasshopper part by part with

that of the cray-fish.

Make diagrammatic representations of imaginary cross-sections
through thorax and abdomen showing the relation of the different
structures : likewise of a sagittal section.

The cricket or cockroach may be substituted for or compared with
the grasshopper.

304. Supplementary Laboratory and Field Work. It

is perhaps inexpedient for students in an elementary course
to make dissections of other representatives of the Arthropoda,
but the common air-breathing forms are so numerous, so
varied, and have such interesting habits and histories, that
they may profitably be used as a basis -for individual field and
laboratory work and to serve in the comparison of homologous
organs in related groups. The fo